The Scotland Yard team arrived in Moscow last night on what would have been Mr Litvinenko’s 44th birthday.

They have officially asked Russian police for their help as they seek the men who met the former KGB colonel before he collapsed.

Lawyers for a jailed former FSB agent have urged the Yard team to visit their client in a top-security prison. They claim that Mikhail Trepashkin has information that could shed light on Mr Litvinenko’s killer.

Mr Trespashkin is alleged to have warned Mr Litvinenko that he was the target of an FSB death squad.

In 2004, Mr Trepashkin was sentenced to four years in prison for disclosing state secrets. Supporters say the case was fabricated to prevent him disclosing evidence of alleged FSB involvement in a series of bombings in Russia that killed 300 people in 1999.

Mr Litvinenko co-authored a book in 2002 that also accused the FSB of carrying out the bombings as a pretext to launch a war in Chechnya that was instrumental in Vladimir Putin’s rise to the presidency.

Police will not say if they intend to approach a number of wealthy Russians who, it is alleged, Mr Litvinenko intended to blackmail.

Julia Svetlichnaja, a London-based academic, told The Observer that Mr Litvinenko confided his plan to her during meetings in London. Ms Svetlichnaja, 33, who is writing a book on the Chechen conflict says she received more than 100 e-mails from the dissident.

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