Scaramella was in a safe house after returning to London from Italy for tests to see if he was contaminated by the same radioactive polonium 210 that was found in Litvinenko's urine. A post-mortem has still to be carried out.

Police said Scaramella is being treated only as a potential witness in the investigation.

Scaramella told last weekend's edition of the Mail on Sunday that in 2003 he made the jump from environmental to intelligence work when he was appointed as a consultant to an Italian parliamentary inquiry, the Mitrokhin commission.

Interviewed while back home in Naples, Scaramella said he met Litvinenko through his work on the commission set up to probe alleged links between former Soviet KGB intelligence and the leftist Italian militants the Red Brigades.

"My work involved a lot of Soviet issues -- the dumping of radioactive waste, which can be detected from space, and the loss of nuclear devices," he was quoted as telling the Mail on Sunday.

Scaramella, who the newspaper says is unmarried with two children, said the commission wanted outsiders to investigate even though he told them he was an expert on nuclear waste, not on security issues.

He said in 2003 he "looked at the operations of the KGB and eastern bloc countries on Italian soil," then in 2004 he led an investigation on the illegal dumping of waste by the mafia in an Italian lake, according to the newspaper.

It also said that in June last year Italian police launched a probe into an alleged plot to smuggle uranium into the country after Scaramella tipped them off. Four people were arrested within a day.

Scaramella said he began his career in 1995 in his native Naples as a lawyer who set up his own company specializing in environmental law.

Naples University -- where reports have said he is based -- cannot say whether he worked or still works there and he does not feature on a public list of academics.

In a 2004 interview with La Repubblica newspaper, he said he was a "security expert" trained in England, France and Belgium, was aged 36 and had two children.

He said he had held a post at San Jose University in the United States and had been recruited by the Central Intelligence Agency (CIA) to go to Colombia to research alleged links between Russian spies and drug trafficking.

Scaramella is said to head up an organization called the Environmental Crime Prevention Programme (ECPP) but information about it is elusive.

The Italian academic met Litvinenko at a central London sushi bar on November 1. Litvinenko fell ill hours later and died on November 23.

Scaramella has denied any involvement in the poisoning following some reports casting suspicion on him and said the meeting was to discuss an alleged Russian secret services "hit list" on which both their names featured.

Italy's Foreign Minister Massimo D'Alema said Scaramella has never been an Italian secret agent, even though he had tried twice to contact the heads of the domestic secret service "who asked him not to seek further contact."

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