Mario Scaramella, the Italian KGB expert who met former Russian spy Alexander Litvinenko the night before he fell ill with radiation sickness, has tested positive for the substance that killed him.
Britain's Health Protection Authority said Friday that a "significant" amount of polonium 210 had been found in Scaramella's urine.
He has been taken to a London hospital for "further tests," the agency said.
It was the first time the deadly radioactive substance had been found in a person linked to Litvinenko.
An autopsy on Litvinenko took place Friday.
British police said they were investigating "a suspicious death from exposure to a radioactive substance".
There has been a wave of speculation about where the polonium 210 came from and how it got to London.
The head of a Russian nuclear facility said Friday that it could not have come from that source.
Scaramella's lawyer said Friday that no formal charges have been made against his client.
A friend of the ex-KGB colonel has stated Litvinenko suspected Scaramella of poisoning him.
"Litvinenko told me that Scaramella was very nervous when they met at the Piccadilly Circus sushi bar," Russian emigre' historian Yuri Felstinki said on Italian TV.
"He didn't eat anything and broke off the meeting suddenly," said Felstinki, who co-wrote a book with Litvinenko, Blowing Up Russia, accusing Russian President Vladimir Putin of ordering Moscow apartment-block bombings in 1999 in order to blame them on Chechen terrorists.
"Alexander was surprised because Mario had come to London to meet him but didn't seem inclined to spend any time with him".
Felstin said his late friend was "sure" that he had been poisoned by the Russian security agency FSB (the KGB's successor) "that the order came from Putin and that Mario Scaramella was the one who carried out the poisoning, because he was an FSB man".
Scaramella's meeting with Litvinenko took place on November 1.
Litvinenko, 44, a close friend of recently slain investigative reporter Anna Politkovskaya, felt stomach pains the next day and was taken to hospital. He died on November 23.
Litvinenko fell ill just days after he accused Putin of ordering Politkovskaya's slaying.
Moscow denies involvement in either case.
Politkovskaya, 48, who accused Putin of sanctioning torture and mass civilian killings in Chechnya, was gunned down in Moscow on October 7.
Scaramella has said he only had a drink in the sushi bar because he had already eaten.
Shortly before the meeting, Scaramella claims he showed Litvinenko threatening e-mails involving St.Petersburg gangsters suspected of carrying out contract killings.
Traces of polonium 210 have been found at the sushi bar, Litvinenko's apartment, the adjacent offices of Russian emigre' billionaire and anti-Putin activist Boris Berezovsky, and the premises of a security and risk-management company, Erinys.
British Prime Minister Tony Blair has said that "no political or diplomatic barrier" would hamper the probe into Litvinenko's death.
He said he would bring up the case with Putin at an "appropriate" time.
Russian Foreign Minister Serghei Iavrov said Friday that Russian authorities had received no inquiry from British police and would deal with the case if and when it was referred to them.
Italian Foreign Minister Massimo D'Alema said Thursday that Scaramella had never had "any relationship with the Italian secret services, apart from two occasions in which he contacted the heads of (civilian intelligence agency) Sisde, who immediately told him not to try contacting them again".
Scaramella describes himself as an academic but has no current institutional affiliations. He reportedly started out as an environmental consultant before becoming a security expert investigating former Soviet espionage and possible nuclear trafficking.
Until earlier this year he worked with Italy's Mitrokhin Commission, set up by Silvio Berlusconi's former centre-right government, which trawled through the Italian part of a massive file of KGB contacts secretly compiled by Moscow archivist Vasily Mitrokhin.
One leftwing former member of the Commission has raised doubts about Scaramella's credentials, noting that the consultant's career resume' was questioned by the panel "so he promptly presented a second, completely different one, which was accepted".
A storm of accusations is raging here in connection with Scaramella's activities as consultant to the Mitrokhin Commission.
In response to a key accusation, Italian Premier Romano Prodi said he was prepared to go to court to defend his reputation.
Italy's leading daily Corriere della Sera reported Friday that Scaramella and Mitrokhin Commission chief Paolo Guzzanti, a senator with ex-premier Silvio Berlusconi's Forza Italia party, discussed digging up evidence that Prodi was "a KGB man".
According to Corriere, which has published a series of phone conversations between the two, Guzzanti ordered the probes after Scaramella claimed he got the tip-off from Litvinenko.
Prodi was one of a number of leftwing politicians cited in the published phone conversations.
All the MPs have reiterated longstanding leftwing charges that the commission amounted to nothing more than a political witch-hunt aimed at discrediting the centre left.
Guzzanti, for his part, declared Friday that "I made these accusations against Prodi publicly".
The Mitrokhin Commission was wound up in March without exposing any leftwing figures - despite months of press speculation led by the Berlusconi family newspaper Il Giornale.
Judicial sources said Friday that Scaramella had been placed under investigation on two counts: breaching secrecy rules in the Mitrokhin affair and possible involvement in suspected uranium trafficking he himself reported.
The head of the probe, which is based in Bologna, stressed Friday that Scaramella "never cited Prodi".
