Without providing details, or explanation, the prosecutor general said that traces of mercury had been found in Moscow and London and that this indicated a link to Nevzlin, who has been accused of using mercury in an alleged 2001 murder attempt against his business partner.

"We are checking out a version in which those who ordered this crime could be the same people who are now on international arrest warrants..., one of whom is deputy Yukos chairman Leonid Nevzlin," the prosecutor said in a statement.

The accusation introduces another colourful and controversial character to a murder enquiry already featuring ex-KGB men, exiled critics of President Vladimir Putin, a mysterious Italian security consultant, and allegedly the world's first use of nuclear material as a murder weapon.

But whether the addition of Nevzlin to the list helps solve the mystery behind Litvinenko's agonising death in a London hospital on November 23 -- a killing that Litvinenko and his friends blame on the Kremlin -- is another question.

Nevzlin, who has been accused of links to several murders in Russia, was one of the key figures in the Yukos oil firm headed by Mikhail Khodorkovsky, formerly Russia's richest man and now serving an eight-year sentence in Siberia for financial crimes.

Israel has refused to extradite Nevzlin to Russia and Nevzlin, like Khodorkovsky himself, claims he is being persecuted for having challenged Putin's increasingly dominating grip on power in Russia.

According to Russian press reports, an attempt is currently underway to extradite Nevzlin from the United States, where he had gone on holiday with his family. Interfax news agency reported he was briefly detained for questioning, but released.

But Alex Goldfarb, an associate of exiled Russian tycoon Boris Berezovsky who was Litvinenko's friend and spokesman, told AFP that the allegation was "total nonsense."

"Accusing Nevzlin, the Kremlin is just trying to cover up," he said. The accusation "just raises the suspicion that the government is trying to hide its responsibility."

So far the Litvinenko investigation has focused on the roles of ex-KGB officer Andrei Lugovoi and former Soviet army officer Dmitry Kovtun.

They met with Litvinenko, a former security services agent who won asylum in Britain, at a London hotel on the day police believe he was poisoned with the deadly radioactive substance polonium-210.

British detectives spent two weeks in Moscow this month trying to unravel the complex web of evidence, which includes traces of polonium in Moscow, Germany, as well as at several locations in London.

Further muddying the waters, Lugovoi and Kovtun have not been seen in public for weeks and are themselves, according to Russian reports, undergoing checks or treatment for radiation poisoning.

Russia's prosecutors classify Kovtun as victim of an attempted murder involving polonium-210, while police in Germany suspect he may have been transporting the deadly material.

The Vremya Novostei newspaper described the linking of Nevzlin to the Litvinenko case as "rather extravagant."