Russian dissident Boris Berezovsky blames the Russian president for the murder of former KGB agent Alexander Litvinenko.
The Kremlin blames its overseas opponents, of whom UK-based Mr Berezovsky is the richest and best-known.
Mr Berezovsky says he himself is in danger of assassination
Mr Berezovsky, a Kremlin insider during the rule of Boris Yeltsin, openly confesses that he is on a mission to bring down President Vladimir Putin "by force" - or at least by means of a bloodless revolution.
But the role of political plotter, and financier of Russian opposition parties, is just the latest in a series of reincarnations that Mr Berezovsky has been through in his 61 years.
His first career was as a mathematician, his second as a used-car salesman, his third as a political kingmaker, nicknamed Rasputin, who assisted Vladimir Putin's rise to power.
He has survived numerous assassination attempts, including a bomb that decapitated his chauffeur, and has been a wanted man in Russia, charged with fraud and political corruption, since 2001.
He now divides his time between an office in Mayfair and an estate in Surrey guarded by former members of the French foreign legion.
Oil and media baron
Among the other Russian dissidents he is allied with in London is the former Chechen warlord, Ahmed Zakayev.
Another was Alexander Litvinenko. Mr Berezovsky bought the ex-spy a house in Muswell Hill and helped him to publicise claims that Mr Putin organised the bombings of apartment blocks in Russia, in 1999, which paved the way for Russia's second military intervention in Chechnya.
Mr Berezovsky made his fortune importing Mercedes cars into Russia in the 1990s, and setting himself up as a middleman distributing cars made by Russia's Avtovaz.
While Avtovaz struggled to survive, Mr Berezovsky nevertheless made millions.
By the mid-1990s, he was one of Russia's leading "oligarchs" who gained control of the country's industrial crown jewels at knock-down prices, in rigged privatisations.
Andrei Lugovoi became security chief for Mr Berezovsky's tv station
As well as taking ownership of the Sibneft oil company, he became the main shareholder in the country's main television channel, ORT, which he turned into a propaganda vehicle for Boris Yeltsin in the run-up to the 1996 presidential election.
It was in 1996 that Andrei Lugovoi, one of the suspects in the Litvinenko case, left the KGB to become head of security at ORT.
Yeltsin's inner circle
Described by critics as the epitome of Russian "robber capitalism", Mr Berezovsky denies having ever taken part in the violence that tainted Russian business in this era.
He took Forbes magazine to court for describing him as the "godfather of the Kremlin" and linking him to the murder of a popular television journalist.
Forbes settled out of court, accepting that the allegations were false.
Mr Berezovsky was at the height of his power in the later Yeltsin years, when he was deputy secretary of Russia's security council, a friend of Boris Yeltsin's daughter Tatyana, and a member of the Yeltsin inner circle, or "family".
Although he helped Mr Putin enter the family, and funded the party that formed Mr Putin's parliamentary base, the new president moved to regain control of the ORT television station, and to curb the political ambitions of Russia's oligarchs.
Mr Berezovsky left Russia for self-imposed exile in the UK at the end of 2000.
An early attempt to promote opposition to Mr Putin, by funding the Liberal Russia party, ended in disaster when its two most prominent members were assassinated.
"I understood [then] that this way of open opposition doesn't work, at least for me. And that's the reason why I decided to choose the other way," he later said.
Without naming Mr Berezovsky, the Kremlin accuses its foreign-based opponents of organising the 2006 assassinations of Alexander Litvinenko and the campaigning journalist Anna Politkovskaya, in order to discredit Mr Putin.
Mr Berezovsky denies the allegation.
He says Mr Putin is prepared to kill anyone that he defines as an enemy of Russia, and that he himself is a target.
That is why the mansion he bought for £10m from former disc jockey Chris Evans is equipped with bullet-proof windows, laser monitors, spy cameras and reinforced steel doors.