Experts probing the death of former Russian spy Alexander Litvinenko have found traces of radioactivity at 12 locations, the home secretary has said.
Mr Litvinenko died last week in a London hospital
Among them are two British Airways (BA) planes. A third one is awaiting checks.
Home Secretary John Reid told Parliament that two Russian aircraft, one of which is currently at Heathrow airport, were also of interest.
The Health Protection Agency said 24 people had been referred to a specialist clinic for tests.
BA is contacting 33,000 passengers from 221 flights. But Mr Reid stressed the public health risk was low.
Mr Litvinenko, an ex-KGB officer and a fierce critic of Russian President Vladimir Putin, died last week of radiation poisoning.
Traces of radioactive polonium-210 were discovered in his body, and more traces of the substance have been found at venues he visited in the capital on 1 November.
Earlier, an inquest into the death of Mr Litvinenko was opened and adjourned at a London court.
Coroner Dr Andrew Reid said the level of polonium-210 in Mr Litvinenko's system indicated it had come from a source "other than a natural one". A post-mortem will be carried out on Friday.
The home secretary said that more locations could be screened for radiation.
"To date, around 24 venues have or are being monitored and experts have confirmed traces of contamination at around 12 of these venues," he said.
Investigations so far have focused on Mr Litvinenko's movements, including his meetings at a West End sushi restaurant and a hotel on 1 November, the day he fell ill.
1 November 20061:
Meeting at Millennium Hotel2:
Meeting at Itsu Sushi bar3:
Home in Muswell Hill4:
Admitted to Barnet hospital17 November5:
Transferred to University College Hospital where dies on 23 November
Other sites where radioactive material has been found:
7 Down Street
25, 58 Grosvenor Street
Two planes at Heathrow Airport
Car in north London
Experts have suggested that polonium-210 could be present in locations, including the planes, either because it leaked from a container or because it was present in people's bodily fluids.
Of those members of the public who were concerned about potential exposure to radiation, 139 had been referred to the Health Protection Agency as a precaution, of which 24 were referred to a specialist clinic.
The HPA said some 53 healthcare staff and a further 140 people who passed through contaminated areas have been asked to provide urine samples.
British Airways said on Thursday evening that one of its two aircraft which had shown contamination had been given the all clear. The HPA said it did not believe passengers on this plane - registration G-BZHA - had been at risk.
Checks are continuing on a second plane at Heathrow, while a third has been ordered to return to the UK from Moscow to be screened.
Officials were also interested in a fourth plane - a Boeing 737 leased to a Russian private carrier, Transaero, which landed at Heathrow airport on Thursday morning. A spokeswoman for the airliner said that no toxic substance had been found on board.
A fifth Russian plane was also said to be of interest, but no further details were given.
"There may be other aeroplanes of which we don't at this stage know, but those are the five that we know of," the home secretary said.
A quarter of the 221 flights made by the three British Airways short-haul 767 aircraft between 25 October and 29 November were between London and Moscow.
Contact with carrier's sweat or urine could lead to exposure
But polonium-210 must be ingested to cause damage
Radiation has very short range and cannot pass through skin
Washing eliminates traces
British Airways said it had contacted 5,500 customers and made information about affected flights available on its website.
"The advice we have been given so far is that the health risk to anyone who has travelled on any of the three aircraft at the centre of the issue around radiation exposure is deemed to be low," it said in a statement.
BA said it was not its role to give medical advice, but referred passengers to medical service NHS Direct.
By midnight on Wednesday, NHS Direct had received 1,707 calls from concerned members of the public.
NHS Direct spokesman Simone Lester said that only passengers who felt unwell should call.
"We will not be in a position to give them more specific advice until the Health Protection Agency has finished its assessment of the potential risk on those flights," she said.
British Airways has set up a special helpline for customers in the UK on 0845 6040171 or +44 191 211 3690 for international calls.
Passengers who travelled on those flights and want further advice are advised to telephone NHS Direct on 0845 4647.