Customers at a restaurant and an hotel visited by a poisoned ex-KGB officer will be tested for the radioactive substance that killed him, said health chiefs.
The Health Protection Agency called for people who had been to the Itsu sushi restaurant or Millennium Hotel in central London on November 1 to come forward.
Its appeal came as the Conservatives indicated that they would ask the Government to make a Commons statement over the affair.
The HPA is taking "extremely seriously" concerns that other people may have been contaminated by the Polonium-210 that led to the death of Alexander Litvinenko in hospital although it made clear the risk was low.
Doctors discovered that he had somehow ingested a large dose of the radioactive substance and samples of it were later found in the hotel and restaurant.
Mr Litvinenko, a former colonel in the Russian security services, visited both places on November 1, the day he was taken ill.
A vocal opponent of Vladimir Putin, Mr Litvinenko, 43, claimed in a statement made public after his death that the Russian president had him poisoned.
Customers who visited Itsu or the Pine Bar or restaurant at the hotel on November 1 were asked to contact NHS Direct.
Scotland Yard's counter terrorism unit is investigating but has not described it as murder. Foreign Office officials have passed on a request via the Russian Ambassador, Yuri Fedotov, asking authorities in Moscow to make available any information which might assist police with their enquiries.
A post-mortem examination of Mr Litvinenko's body has been delayed while a risk assessment is carried out to see if it is safe to perform the procedure and what precautions may be necessary.
