Rasputin, the Russian monk who became the confidant of Alexandra, the Tsarina, and her husband, Tsar Nicholas II, was killed by a British agent, according to a documentary to be broadcast next month.

An investigation into his death in 1916 has concluded that he was murdered not as had been supposed by disaffected Russian aristocrats but by Oswald Rayner, a member of the Secret Intelligence Bureau who was working at the Russian court in St Petersburg.

Richard Cullen, a retired Scotland Yard commander who has been studying the case with Andrew Cook, an intelligence historian, says that a new forensic analysis and an examination of official records helped him to reach his conclusion.

"I am 99.9 per cent certain of this," said Mr Cullen, whose findings will be broadcast in the BBC2 Timewatch programme on October 1."There is a fair weight of evidence to show that Rayner was the man. We have conclusive proof that the previously accepted versions of events are fabrications."

Rasputin claimed to have"mystical powers", which gained him the confidences of first the Tsarina, Alexandra - who thought that he could cure her haemophiliac son - and Tsar Nicholas. But he was highly unpopular among courtiers and his killers evaded trial, publishing memoirs that described the murder in detail.

Now it is claimed that the SIB wanted to kill Rasputin, who was hoping to broker peace between Russia and Germany, because of his influence over the Tsar. The fear, according to Mr Cullen, was that if such a deal had been agreed in 1916, 350,000 German troops would have been freed to fight the Allies on the Western Front.

According to Timewatch Rayner was known to be in St Petersburg in December 1916 when Rasputin died. A close friend from university was Prince Felix Yusupov, at whose sumptuous palace the murder took place.