Yuri Shvets, a former spy now based in America, claims Litvinenko had been doing due diligence work for a British company on the official, who was facilitating a business deal.

Shvets believes Litvinenko had acquired a damaging eight-page dossier with details on the official that may have ruined a multi-million-pound deal with the British company.

The claims shed new light on the activities of Litvinenko, who died on November 23 after being poisoned with polonium-210, a radioactive substance.

His death has been the subject of several theories, including claims that he was murdered in a Kremlin plot to silence his criticism of Vladimir Putin’s regime.

Shvets is a former KGB major who now works from Washington, advising businesses on corruption and security in the former Soviet Union.

He has been interviewed by detectives from Scotland Yard. He gave his first full interview last week to his friend Tom Mangold, a journalist, in a programme for BBC Radio 4.

Shvets says Litvinenko came to him for help after a British security company had offered him a $100,000 contract to do due diligence work on five Russian figures.

One of the five, whom Shvets refused to name, is said to be a powerful Kremlin official.

Litvinenko acquired the eight-page dossier on September 20. Shvets says Litvinenko showed the dossier to Andrei Lugovoi, another former Russian agent, two weeks later.

This, Shvets believes, was a mistake because he claims Lugovoi probably tipped off the official about the dossier.

“I believe the dossier was the trigger for the assassination,” he said.

Lugovoi met Litvinenko at the Millennium hotel in London on November 1, the suspected day of the poisoning. Lugovoi has denied any involvement in the murder and is himself contaminated with polonium.

Scotland Yard detectives were present at interviews with Lugovoi and others during a visit to Moscow last week. The British officers were not allowed to put any questions, however. Russian prosecutors conducted the interview.

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