Ex-Russian agent Alexander Litvinenko died in a London hospital on 23 November 2006 from radiation poisoning.
Here are some of the key events leading up to and since his death.
1 NOVEMBER 2006
Mr Litvinenko meets two Russian men at a London hotel - one a former KGB officer.
He also meets academic Mario Scaramella at a sushi bar where he is said to have received documents about the death of Russian journalist Anna Politkovskaya.
Several hours after his meetings, Mr Litvinenko complains of feeling sick and spends the night vomiting.
After three days of sickness and stomach pains Mr Litvinenko is admitted to Barnet General Hospital, north London.
On the BBC Russian Service, Mr Litvinenko describes being in "very bad shape" after a "serious poisoning".
Mr Litvinenko is transferred to the University College Hospital, in central London, as his condition worsens. He is placed under armed police guard.
It is reported Mr Litvinenko was poisoned with thallium, a highly toxic chemical once used to poison rats.
Mr Litvinenko is moved to intensive care. Pictures are released of the ex-agent in hospital, showing how he has suffered dramatic weight and hair loss.
Scotland Yard's counter-terrorism unit takes over the investigation into what made him ill. Police say they are treating the case as a suspected "deliberate poisoning" but await toxicology test results.
The Kremlin dismisses allegations Russia's government poisoned Mr Litvinenko because of his criticisms of its policies as "sheer nonsense".
There is confusion over what happened to make Mr Litvinenko ill.
Professor John Henry, a toxicologist, says Mr Litvinenko may have been poisoned with "radioactive thallium".
Doctors directly responsible for his treatment say Mr Litvinenko's illness is unlikely to have been caused by ordinary thallium poisoning.
Mario Scaramella tells a news conference in Italy that he had met the Russian in a Sushi bar earlier in the month to discuss e-mail threats they had received.
Mr Litvinenko is described as "critically ill".
Dr Geoff Bellingan, director of critical care at University College Hospital, rules out thallium as the cause of his sickness.
Russia's foreign intelligence service denies involvement in the apparent poisoning of Mr Litvinenko.
The ex-agent has a heart attack overnight.
Mr Litvinenko dies in intensive care.
Scotland Yard says it is now investigating "an unexplained death".
A statement made by Mr Litvinenko before he died is read out by his friend Alex Goldfarb outside University College Hospital, London.
In it he accuses Russian President Vladimir Putin of involvement in his death and says his killer was "barbaric and ruthless".
Protest from around the world "will reverberate, Mr Putin, in your ears for the rest of your life," he said.
Friend Andrei Nekrasov says that just hours before Mr Litvinenko fell unconscious, he told him: "The bastards got me but they won't get everybody."
Mr Putin, at a press conference, says Mr Litvinenko's death was a tragedy, but he saw no "definitive proof" that it was a "violent death".
Mr Litvinenko's father Walter tells reporters his son was killed by a "tiny little nuclear bomb".
Health experts say they believe Mr Litvinenko was deliberately poisoned by radioactive matter, believed to be polonium-210.
Police find traces of radioactive material at the sushi bar and the hotel where the former spy had meetings on 1 November, and at his north London home.
Tests are to be carried out on people who may have come into contact with Mr Litvinenko, including those who were at the Itsu sushi bar in London.
Police were searching the sushi bar and a bedroom at the Millennium Hotel.
Hundreds of people contact the NHS Direct hotline to seek advice about potential radiation poisoning.
In an emergency statement in the Commons, Home Secretary John Reid tells MPs the Russian ambassador was called to the Foreign Office at the end of last week to "convey to the Russian authorities our expectation that they should be ready to offer all necessary co-operation to the investigation".
Police confirm traces of radioactive polonium-210 have been discovered at two more central London addresses - Grosvenor Street, Mayfair and Down Street in West London.
Three people linked to the central London venues Mr Litvinenko visited on 1 November are referred to a clinic for radiological tests, after reporting possible radiation symptoms.
The Health Protection Agency (HPA) announces that eight people have been referred for tests after contacting NHS Direct.
Mr Scaramella tested negative for polonium, BBC Panorama learned.
A total of 1,121 people have called NHS Direct over the alert and 68 have been assessed as needing follow-up help, the HPA says.
It emerges Italian academic Mario Scaramella, who met Mr Litvinenko on the day he fell ill before returning to Italy, has travelled back to the UK to be tested.
Sources say police will also interview Mr Scaramella as a potential witness in the inquiry into the death.
Scotland Yard later confirms it is checking a further two addresses - the Sheraton Park Lane hotel and 58 Grosvenor St for traces of polonium-210.
Prime Minister Tony Blair says no "diplomatic or political barrier" will stand in the way of the police investigation.
Traces of a radioactive substance are found on two British Airways planes at Heathrow Airport.
The Boeing 767s, plus a third in Moscow, are to be tested and BA says it plans to contact the thousands of passengers who travelled on the European flights affected, including the London to Moscow route.
Meanwhile, Mr Scaramella says tests have cleared him of exposure to the deadly radioactive poison.
The HPA reveals that of the 4,000 staff at the two hospitals where the former spy was treated, 160 were assessed for possible exposure.
Foreign Office Minister Kim Howells tells MPs that Russian dissidents living in Britain "have the right to expect to be able to walk our streets without the fear of being murdered".
Home Secretary John Reid tells MPs traces of radioactivity had been discovered in 12 locations, as well as the two British Airways planes.
Mr Reid tells MPs that two Russian aircraft, one of which is currently at Heathrow airport, are also of interest.
The HPA says 24 people have been referred to a specialist clinic for tests.
An inquest into the death of Mr Litvinenko is opened and adjourned at a London court.
Italian academic Mario Scaramella tests positive for a significant amount of polonium-210, although he has not suffered any symptoms of poisoning.
It is also revealed that Mr Litvinenko's wife Marina has tested positive for the alpha particle-emitting substance, although she is not admitted to hospital and is not believed to be in any danger.
A room is sealed off at Ashdown Park Hotel in Sussex.
In the wake of the alleged poisoning of former Russian Prime Minister Yegor Gaidar radiation tests are carried out in the Republic of Ireland.
Nine British police officers travel to Moscow to pursue their investigation into Mr Litvinenko's death.
Further tests are carried out at a number of locations in central London.
The Parkes Hotel in Knightsbridge and an office in Cavendish Place are being examined.
Tests carried out by the Health Protection Agency (HPA) at the Best Western Hotel in Shaftesbury Avenue, London, find nothing of concern to public health.
The HPA say just over 3,000 people in the UK have now called the NHS Direct line since the radiation scare, with 179 being followed up for further investigation.
So far, 27 people have been referred as a precaution to a specialist outpatient clinic for radiological exposure assessment.
And a total of 70 urine samples, mainly from medical staff and ambulance workers, have been tested and found to be negative.
Russia's prosecutor general Yuri Chaika says he will not extradite suspects in the poisoning of Mr Litvinenko to Britain.
Mr Chaika says any trial of a Russian citizen must take place in Russia and that it would be "impossible" for British officers to arrest Russians in their home country.
Russian prosecutors say they intend to question former KGB bodyguard Andrei Lugovoi, who met Mr Litvinenko in London on 1 November.
A room at the British Embassy in Moscow is tested as a precaution, after Mr Lugovoi's visit to the building last week. Mr Lugovoi is reported to be in hospital.
British police say they are now treating the death of Alexander Litvinenko as murder.
Radiation is found at the British embassy in Moscow following precautionary tests on the building.
The embassy says there is no danger to public health.
Mr Scaramella is discharged from University College Hospital. Doctors say he had not shown any sign of illness.
The Health Protection Agency confirms staff at the Itsu sushi restaurant have all tested negative for radiation.
Russia says it is investigating the attempted murder of Dmitry Kovtun, a contact of Mr Litvinenko who met him at the Pine Bar in London's Millennium Hotel on 1 November.
Marina Litvinenko led mourners at her husband's funeral
Reports that Mr Kovtun is in a critical condition spark speculation he too may have been poisoned.
A lawyer linked to him denies he is unwell.
All seven bar staff working at the Pine Bar at the time test positive for low levels of polonium-210.
Meanwhile, about 50 mourners - including his wife, son and parents - see Mr Litvinenko buried at North London's Highgate Cemetery.
More than 200 people who visited the Pine Bar on 1 November will be offered tests for radiation, it emerges.
Police in Germany say they have found indications of radiation in two properties apparently used by Dmitry Kovtun - the Hamburg flat of his ex-wife, and her mother's home outside the city.
Officials in Moscow say Russian police may travel to Britain as part of their parallel investigation.
Mr Litvinenko's widow Marina says she believes the Russian authorities could have been behind his murder in an interview with the Mail on Sunday.
She tells the paper: "Obviously it was not Putin himself, of course not."
But she says what President Putin "does around him in Russia makes it possible to kill a British person" in Britain.
The Metropolitan Police says two of the 26 police officers closely involved in the Litvinenko inquiry have tested positive for "relatively small" levels of polonium-210, within safety limits.
A British detective arrives in Hamburg to be briefed by German police on their investigation into traces of polonium-210 found at various residences.
Four people close to Dmitry Kovtun also test positive for the radioactive substance. They are his ex-wife, her partner and two children.
And in Moscow, British detectives and officials from the Russian Prosecutor General's office reportedly interview ex-KGB bodyguard Andrei Lugovoi, who is currently in hospital.
Ex-spy Yuri Shvets claims Mr Litvinenko was murdered because of information he held on a powerful Kremlin figure.
Mr Shvets, a former business associate of the murdered former KGB man, says he was poisoned after the dossier which had damaging details was deliberately leaked to the high-ranking Moscow figure.
Mr Scaramella is arrested in Naples, Italy, as part of an investigation into arms trafficking and violating state secrets. Scotland Yard say the arrest is not connected to the investigation into Mr Litvinenko's death.
Russia's chief prosecutor says he is investigating whether a former boss of oil firm Yukos, Leonid Nevzlin, may be linked to the death of Mr Litvinenko.
The prosecutor-general's office says Mr Nevzlin is among ex-Yukos executives wanted internationally "over serious crimes".
A spokesman for Mr Nevzlin reportedly says the suggestion concerning Mr Litvinenko is "ridiculous".
3 JANUARY 2007
Two more people in London test positive for polonium-210, health officials announce.
The people affected are a staff member at the Best Western Hotel, Piccadilly, and a guest at the Millennium Hotel's Pine Bar in Mayfair, both in central London.
It takes the total number of people affected by the radioactive substance to 12.
Polonium-210 is detected at another restaurant - the Pescatori in Mayfair, central London - health officials say.
Police had asked the Health Protection Agency to monitor the establishment as it was "linked to the Litvinenko police investigation".
The agency says there is no reason for public health concern over the discovery.
The Health Protection Agency reveals that 120 of the 596 people tested for polonium-210 showed traces of radiation.
But it says just 13 of those who tested positive following Mr Litvinenko's death are deemed to have a health risk, and that the long-term risk is very small.
The agency says it has identified 450 people worldwide who may have been affected by radiation, and is working with 48 different countries on the matter.
Russian prosecutors ask the UK for permission to question more than 100 witnesses over the poisoning of Mr Litvinenko.
Russia's deputy chief prosecutor Alexander Zvyagintsev also says Russian investigators want to examine "dozens" of places in Britain in connection with the former agent's death.
British detectives investigating Mr Litvinenko's death have asked to return to Moscow to carry out further inquiries, it is reported.
Russian's prosecutor general Yuri Chaika is quoted as saying he does not rule out another visit from UK investigators, after Russian investigators have visited Britain.
BBC One's Panorama programme reports there may have been multiple attempts to kill Mr Litvinenko before he died.
It says the first attempt to poison him may have come two weeks before he met Mario Scaramella in a sushi bar on 1 November.
The programme says it may have been at the same restaurant, but when Mr Litvinenko met former KGB men Andrei Lugovoi and Dimitri Kovtun on 16 October.
Panorama adds that although it was widely reported that Mr Scaramella had tested positive for polonium-210, it has discovered his initial test results were inaccurate. Subsequent tests proved negative.
Mr Litvinenko's widow Marina tells the programme her husband's poisoning could not have been carried out without Russian President Vladimir Putin's knowledge, but Mr Putin's spokesman Dimitry Peskov vehemently denies this.
Andrei Lugovoi laughs off reports London may soon seek his extradition.
Mr Lugovoi denies any involvement in Mr Litvinenko's death
He tells the BBC he has not read a Guardian newspaper report which claims Scotland Yard has allegedly collected enough evidence for him to stand trial in the UK.
The former KGB officer has persistently denied any involvement in the murder.
He goes on to refuse to comment on the possibility of being extradited to Britain or that he could be swapped with the London-based Russian exile Boris Berezovsky, a leading opponent of President Vladimir Putin.
Andrei Lugovoi and Dmitry Kovtun, the two Russian businessmen questioned by British police over the poisoning of Mr Litvinenko, deny being suspects in the case.
They tell a Russian TV station that UK press reports describing them as suspects were "a lie".
Mr Litvinenko had met the men hours before falling ill.
In their first joint TV interview, the men tell Russia Today they are witnesses, not suspects.
Mr Lugovoi and Mr Kovtun, both businessmen and former members of the Russian security services, were taken to hospital with suspected radiation poisoning in December.
Scotland Yard hands a file on the investigation into the death of Mr Litvinenko to the Crown Prosecution Service.
The BBC's security correspondent Frank Gardner says police sources have told him that the "finger of suspicion" pointed "clearly" at former KGB officer Andrei Lugovoi.
But Mr Lugovoi has said he was also a victim of radiation poisoning.
Police say they cannot reveal the contents of the file.
Prosecutors will consider whether there is sufficient evidence to charge anyone over Mr Litvinenko's death.
The Home Office confirms Russian detectives investigating Mr Litvinenko's death have asked for permission to come to the UK.
The Metropolitan Police said a decision on whether to agree to the request would be made "in due course".
Boris Berezovsky breaks his silence to tell BBC's Newsnight that Andrei Lugovoi was responsible for the death of his close friend, Mr Litvinenko.
He says Mr Litvinenko told him: "I think Lugovoi is involved in my poison," but the former KGB agent denies any involvement.
Alexander Gusak, a former head of the FSB - the successor of the KGB - says Mr Litvinenko was a "direct traitor" for betraying other Russian agents to British intelligence.
He told BBC's Newsnight show that in Soviet times Mr Litvinenko would have been sentenced to death, and under current law would face up to 20 years in prison for treason.
Mr Gusak says one of the agents who believed he had been exposed by Mr Litvinenko offered to assassinate the former spy.
He also confirms claims made by Mr Litvinenko of a plan to kill Russian oligarch, Boris Berezovsky, in 1997.
Two more people test positive for polonium-210, the Health Protection Agency announces.
One of them had been inside the Millennium Hotel in Mayfair while the other was a member of staff at the Sheraton Park Lane Hotel on the day Mr Litvinenko visited both and was contaminated.
It brings the number of people affected by polonium-210 and facing possible health risks to 15.
Russian billionaire, Boris Berezovsky, who lives in exile in Britain, is questioned about the death of ex-spy Alexander Litvinenko.
He said he agreed to be questioned, but insisted on security precautions for his own safety.
The wife and friends of Mr Litvinenko launch a justice foundation in his name.
His widow Marina, close friend Alex Goldfarb and Boris Berezovsky hope the foundation will assist investigations into his death.
Andrei Lugovoi should be charged with the murder of Mr Litvinenko, the director of public prosecutions recommends.
Sir Ken Macdonald says that Mr Lugovoi, who denies involvement in the death, should face trial.
Speaking at a news conference in Moscow, Mr Lugovoi said Mr Litvinenko was not his enemy and that he had nothing against him. He claimed that Mr Litvinenko was a British spy.
Mr Lugovoi also said the British secret service tried to recruit him to provide "compromising information" on President Putin.
The widow of Mr Litvinenko dismisses claims that British secret services were involved in his death.
Marina Litvinenko says that accusations made by Mr Lugovoi, suspected by Britain of poisoning her husband, were "nonsense".
The Russian prosecutor-general rules out extraditing Mr Lugovoi.
"Extradition is out of the question, because it contradicts our constitution," Yuri Chayka is quoted by the Itar-Tass news agency as saying.
Russia launches a spying case in connection with statements made by Mr Lugovoi which accuse Mr Litvinenko and businessman Boris Berezovsky of having contacts with the UK secret service.
The Federal Security Service is behind the investigation but no suspects have been named by the authorities.
Russia officially refuses a UK extradition request for Mr Lugovoi.
The Russian Prosecutor General's Office said the constitution did not allow for the extradition of its citizens.
The government announces it is to expel four diplomats from the Russian embassy in London after Moscow refused to extradite Mr Lugovoi.
Foreign Secretary David Miliband also said co-operation with Russia on a range of issues was under review.
Moscow warns the UK to expect "serious consequences" after the expulsion of its diplomats and says the British authorities
will be "adequately informed" of its feelings in time.
The Foreign Office said retaliation by Russia was not "justified."
Russian deputy foreign minister Alexander Grushko also said the diplomatic expulsions could jeopardise co-operation between the two nations on counter-terrorism.
Russia announces it is to expel four British embassy staff, mirroring the UK's decision to expel four staff from the Russian embassy in London.
The four are given 10 days to leave, and Moscow says it is also to review visa applications for UK officials.
Foreign ministry spokesman Mikhail Kamynin says co-operation in counter terrorism will no longer be possible.
He also says Moscow will not apply for any UK visas for Russian officials.
The Russian Embassy confirms the four expelled diplomats have left the UK.