Seven workers at a hotel where murdered ex-KGB spy Alexander Litvinenko held a meeting have tested positive for low levels of radioactive polonium-210.
Mr Litvinenko died on 23 November in a London hospital
The Health Protection Agency is now asking anyone who was at the Millennium Hotel's Pine Bar between 31 October and 2 November to contact NHS Direct.
It stressed the risk to the general public was likely to be "very low".
A Russian businessman who met Mr Litvinenko in the London bar is critically ill, according to reports.
On Thursday, about 50 friends and family members of Mr Litvinenko attended his funeral at Highgate Cemetery in north London.
Mr Litvinenko allegedly met an associate, Dmitry Kovtun, and his business partner Andrei Lugovoi, also a former spy, at the Millennium Hotel on 1 November.
Hours later former KGB agent Mr Litvinenko fell ill. He died on 23 November.
The other two men are also reported to be ill.
Russia's Interfax news agency claimed that Mr Kovtun was taken ill after he was questioned by British and Russian investigators
His condition was described as critical, although earlier the agency reported he had fallen into a coma. A lawyer has denied there was any deterioration in his condition.
Mr Lugovoi is also in hospital but his condition is unclear.
All seven bar staff from the Pine Bar who were checked, tested positive for polonium-210.
The HPA said that as far as the workers were concerned there was "no health risk in the short term and in the long term the risk is judged to be very small on the basis of initial tests".
However, the concern is that they may be at a very small increased risk of cancer in the long term.
The HPA added: "All of them are assessed to have had intakes lower than the adult family member of Mr Litvinenko for whom tests also identified they had been exposed to polonium-210.
"Our first priority has been to speak with these staff members and agency experts have been explaining the results and answering any questions they may have."
It said the staff should carry on with their daily lives "as normal" and it was "extremely unlikely" that people around them were at any risk.
However, the HPA would not speculate on how the members of staff may have been infected.
HPA chief executive Pat Troop added: "Compared to Mr Litvinenko, they have a fraction of what he was exposed to. It's nothing like what he had.
"We appreciate that this isn't an easy message to hear, and there will be anxiety among the public who were there, which is why we are offering support and testing."
The agency said it was assessing staff who worked in the hotel's Pine Bar between 31 October and 2 November.
Members of the public who were in the bar on these dates can contact NHS Direct on 0845 4647.
Around 250 people who were in the bar on November 1 have already contacted the advice line, and do not need to call again, the HPA said.