Senior officers have briefed ministers on the man’s identity and on how he made several trips to London in the fortnight before the former Kremlin spy fell ill last month. Nine officers from Scotland Yard’s Counter-Terrorism Unit have been given permission to travel to Moscow by the Russian authorities and told that they will be shadowed by local police during their investigations.

Andrei Lugovoy, a former KGB agent turned millionaire entrepreneur, is believed to be among five men the police want to meet. He made three trips to London from October 16 and met Mr Litvinenko four times in the days before the latter was fatally poisoned with polonium-210.

Mr Lugovoy has told The Times of sharing drinks and dining with Mr Litvinenko, including a meal they had in October at the same Piccadilly sushi bar where detectives believe the Russian dissident was poisoned on November 1.

The two men first met in 1997 when Mr Lugovoy went to work for a television station run by the exiled billionaire Boris Berezovsky. Mr Berezovsky, who is now based in London, also employed Mr Litvinenko.

Security sources say that Mr Lugovoy is reported to retain close contacts within Russia’s Federal Security Service, the FSB.

Mr Lugovoy denies involvement in Litvinenko’s death, and said that he would welcome the chance to clear his name.

Traces of polonium-210 have been found at two Mayfair hotels in which Mr Lugovoy stayed and where he entertained Mr Litvinenko, as well as on a British Airways plane on which he travelled.

He also visited the West End office of Mr Berezovsky, another contaminated site. Mr Lugovoy was one of the last people to talk to the 43-year-old dissident before he collapsed on the night of November 1 at his home in Muswell Hill, North London.

Last night Alex Goldfarb, a close friend of the victim, revealed that on his death bed Mr Litvinenko voiced his suspicions about the former FSB agent. Mr Goldfarb said yesterday that his friend did not want to publicise details of his encounters with Mr Lugovoy and some of his associates, in the hope that he would recover and lure these men back to London when he was better.

Mr Lugovoy and two business colleagues, Dmitri Kovtun and Vyacheslav Sokolenko, were in London on November 1 to watch CSKA Moscow play Arsenal in a Champions League tie. All three met Mr Litvinenko before the game at the Millennium Hotel, in Grosvenor Square. Police are investigating whether the football match was used as a cover by those involved in the poison plot to come to London.

Mr Lugovoy says that he has been in touch with Scotland Yard and is looking forward to having the opportunity to clear his name. At the weekend he suggested that he and his colleagues were being framed to draw police away from the real perpetrators, whose identity he says he does not know.

Mr Lugovoy claimed that he and his wife and children had all tested positive for low-level exposure to polonium-210, saying that the poisoner may have tried to kill him as well.

His Pershin company, with interests in private security, soft drinks and wine, is said to be worth £100 million, raising questions about why such a successful executive would be involved in such an affair.

There are reports that Tony Blair is concerned about the diplomatic damage with Moscow. President Putin is furious at the continuing allegations of the Kremlin’s involvement in the Litvinenko affair.

The Moscow link was reinforced over the weekend by reports that British officials had questioned another former Kremlin spy now in hiding in Washington.

The latest theory to emerge is that he was planning to blackmail several wealthy Russians about their private lives and business dealings.

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