Alexander Litvinenko, the poisoned former Russian agent, told the Italian academic he met on the day he fell ill that he had organised the smuggling of nuclear material out of Russia for his security service employers.
Mario Scaramella, who flew into London yesterday to be interviewed by Scotland Yard officers investigating Mr Litvinenko's death, said Mr Litvinenko told him about the operation for the FSB security service, the successor to the KGB.
Police said that Mr Scaramella, who met Mr Litvinenko at a sushi bar in London on 1 November to discuss a death threat aimed at both of them, was a potential witness. He was being interviewed at a "secure location" in London but was not in custody.
The Health Protection Agency said that eight people had been referred to a clinic in London for tests for exposure to polonium-210, the radioactive substance that killed Mr Litvinenko. It declined to say whether Mr Scaramella was among them.
A post-mortem examination will be carried out on Mr Litvinenko on Friday.
In an interview with The Independent shortly after the poisoning became public, Mr Scaramella said that Mr Litvinenko, a friend and professional contact since 2001, told him he had masterminded the smuggling of radioactive material to Zurich in 2000. There have long been concerns that turmoil in Russia and other former Soviet states after the fall of Communism created an international black market in radioactive substances.
The operation would have been one of the last carried out by Mr Litvinenko while still an FSB officer, in a unit tackling organised crime and smuggling. He fled Russia for London that year after the FSB began investigating him for corruption - charges which he claimed were invented as revenge for his decision to expose an FSB plot to assassinate the Russian oligarch Boris Berezovsky.
Friends of Mr Litvinenko, a critic of the Russian President, Vladimir Putin, said last night that they were unaware of his involvement with any smuggling for the FSB. Alex Goldfarb, an ally of Mr Berezovsky, said: "He did not mention anything about nuclear material while serving with the FSB."
Mr Litvinenko died on Thursday last week after publicly accused Mr Putin of ordering his poisoning.
Mr Scaramella, an academic and examining magistrate based in Rome and Naples, had been due to meet Mr Litvinenko on 10 November in London, but brought the meeting forward at short notice on 1 November. The Itsu restaurant in Piccadilly, where traces of polonium-210 have been found, is thought to be the first location visited by Mr Litvinenko on 1 November.
Later he met two Russian business associates at a Mayfair hotel and visited the nearby offices of Mr Berezovsky and a security firm, where polonium traces have also been found. Last night police confirmed that they were searching the five-star Sheraton Park Lane Hotel in Mayfair as well as an office building in the West End.
Mr Scaramella has denied any involvement in his friend's death and derided suggestions that he was himself a Russian agent.He claims that he has long been involved in investigating the smuggling of radioactive material by the KGB and its successors. He claimed last year that Soviet destroyers had laid 20 nuclear torpedoes in the Bay of Naples in 1970, where they remain.
Mr Berezovsky, the exiled Russian billionaire visited almost daily by Mr Litvinenko, said: "I am deeply saddened at the loss of my friend. I credit him with saving my life and he remained a close friend and ally."
Russian authorities again denied involvement in the case, while Tony Blair vowed that there would be no "diplomatic or political barrier" to the inquiry.