A contact and the wife of dead former spy Alexander Litvinenko have both tested positive for polonium-210, the substance found in the Russian's body.
Mario Scaramella met Mr Litvinenko the day he fell ill
Italian Mario Scaramella is not thought to be suffering physical symptoms but the amount found is "likely to be of concern for [his] immediate health".
Mr Litvinenko's wife, Marina, has also been found to have traces of the substance but is not currently ill.
She is reported to be "very slightly contaminated" and is not in hospital.
A spokesman for the Health Protection Agency said: "The levels are not significant enough to result in any illness in the short term and the results are reassuring in that any increased risk in the long term is likely to be very small."
Mr Scaramella met Mr Litvinenko at a sushi restaurant, Itsu, in central London on 1 November, the day the former KGB agent fell ill.
The Italian has now been admitted to London's University College Hospital for further tests.
Dr Keith Patterson, of University College Hospital, said: "Tests have detected polonium-210 in Mr Scaramella's body, but at a considerably lower level than Mr Litvinenko.
"He is currently well and shows no symptoms of radiation poisoning. He is receiving further tests over the weekend."
The post-mortem examination on Mr Litvinenko, a former KGB agent, has been completed.
Those present at the examination at the Royal London Hospital, in east London, wore protective clothing to avoid contamination by traces of the polonium-210 isotope. Results may not be available for several days.
The BBC understands Mr Litvinenko's family have been told he will have to be buried in a sealed coffin, and if they wanted to have a cremation they would have to wait 22 years.
Mr Scaramella is involved in an Italian parliamentary inquiry into KGB activity and was sufficiently worried by the contents of an e-mail to ask for advice from Mr Litvinenko.
The e-mail said that he, Mr Litvinenko and an Italian senator, Paolo Guzzanti, were possible targets for assassination.
Friends of Mr Litvinenko believe he was poisoned because of his criticisms of the Putin government.
The Kremlin has denied any suggestion it was involved in any way as "sheer nonsense".
Mr Litvinenko died last week in a London hospital
Mr Scaramella was taken to University College Hospital in an ambulance.
A room was also sealed off at Ashdown Park Hotel, in Sussex. It is thought this is in connection with the investigation into Mr Scaramella.
UKIP MEP Gerard Batten, an acquaintance of Mr Scaramella, said he had seemed fine when they last spoke.
"The last time I spoke to him, on Sunday I think, his worry about his own contamination had been allayed - he thought he was ok."
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
Mr Guzzanti has indicated he fears for his life and is contacting Italian authorities to check if he has been poisoned.
Mr Litvinenko, an ex-KGB officer and critic of Russian President Vladimir Putin, died last week of radiation poisoning attributed to polonium-210.
The investigation into the spy's death has unearthed traces of radiation at 12 locations, including two BA planes.
British Airways is contacting 33,000 passengers from 221 flights, but the airline and the government have stressed any risk to public health low.
On Friday the British Embassy in Moscow said there was no information to suspect any link between Mr Litvinenko's death and former prime minister of Russia Yegor Gaidar, who was taken ill in the Republic of Ireland.
But RTE news reported that the Radiological Protection Institute of Ireland is checking the National University of Ireland Maynooth, County Kildare, and the James Connolly Memorial Hospital in Blanchardstown, Co Dublin for the presence of polonium.
The university has done its own parallel tests, which it says were negative.
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.