The intelligence operations going on in the background were run by Alley, Captain John Dymoke Scale, the subsequent recruiter of Sidney Reilly, and another MI1c officer, Lieutenant Oswald Rayner, who had been sent out in November 1915 with Major Henry Vere Benet to carry out an extensive "censorship" of telegrams and mail examined in total collaboration with the Russian authorities, while sharing the intelligence it produced on a rather more selective basis. Some of the most productive material came on Scandinavian shipping companies taking goods into or out of Germany through the Royal Navy economic blockade.

Rayner had been born in Smethwick, just west of Birmingham, where his father was a draper. He eventually got himself a job teaching English in Finland, which was then an autonomous grand duchy of Russia. He appears to have been adopted by a couple with whom he became friendly and who acted as his benefactors funding an education at Oxford, where he studied modern languages and made influential new friends, including Prince Feliks Yusupov, a member of what was reputed to be the richest family in Russia. A brief career as a junior Paris correspondent for The Times was followed by a move into the civil service as private secretary to Sir Herbert Samuel, the Postmaster-General, through whom he came to know David Lloyd George, a future Prime Minister, who was then Chancellor of the Exchequer. When war broke out, Rayner sought a commission in the army and with his fluency in French, Gernian and Russian was swiftly recognised as having intelligence potential. He was commissioned into the Special List and assigned to Cumming. After a brief period working at Whitehall Court, he was sent to Russia where he shared an apartment with Benet and Alley.

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.

Yusupov claimed a prominent role in the death. In his account, until now the accepted version of events, he lured Rasputin to his palace and fed him cyanide-laced cakes. When these did not take effect, he got a gun from the study where his co-conspirators waited, and fired at the monk's heart.

He returned to the murder scene an hour later and was horrified to discover that Rasputin was still alive. The monk leapt to his feet, attacked Yusupov, then fled into the courtyard where he was gunned down by another conspirator, Vladimir Purishkevich.

Yusupov wrote that the next day he dined with Rayner, who "knew of our conspiracy and had come in search of news".

The BBC documentary says that modern forensic evidence contradicts this account. Post-mortem photographs of Rasputin show a mysterious third bullet wound in the centre of his forehead. The precise positioning of this, the fatal shot, suggests that it was the work of a professional killer. It was also fired at close range, yet Purishkevich shot Rasputin from behind, at a distance.

The three bullet holes are of different sizes, and forensic scientists have now determined that the bullets were fired from three different guns.

Mr Cullen concludes that there was a third gunman, and that this was Rayner, who knew about the plot, was at the palace and wanted Rasputin dead.

Photographs of the palace taken after the body was discovered show a long, straight line of blood across the courtyard, ending in a pool of blood near a gate where a car was waiting.

Mr Cullen surmises that after being shot by both Yusupov and Purishkevich, Rasputin was carried across the courtyard but that before they reached the car, which was to have disposed of the body, he showed faint signs of life. Rayner promptly dispatched him with a bullet to the head.

Rayner's involvement was kept secret by his superiors and by the Russian conspirators, who were eager to gain the glory themselves. Further weight to this new version of events is a copy of a memo sent between Rayner's two superiors in St Petersburg, John Scale and Stephen Alley, who were away at the time of the murder. The memo reads: "Although matters have not proceeded entirely to plan, our objective has clearly been achieved. Reaction to the demise of 'Dark Forces' [a codename for Rasputin] has been well received by all, although a few awkward questions have already been asked about wider involvement. Rayner is attending to loose ends and will no doubt brief you on your return."

If Rayner was the killer he never spoke about what he had done. He burnt all his papers and took the secret to his grave in 1961. He left Russia before the end of the war and in 1920 worked for The Daily Telegraph as Finnish correspondent.

He spent his final years in the village of Botley, Oxfordshire, where he was a fund-raiser for his local church. His only son, John Felix Rayner, named after his friend Yusupov, died in 1965.

The presence of these three men was in my view necessary to provide witness to Rasputin's death and, after the event, to portray the still fondly held view that this was sonic romantic plot to save Mother Russia carried out by a member of the imperial family, the heir to a fortune and a member of the Duma. Lazovert was there to pronounce life extinct at an appropriate time. Dmitrii, Yusupov and Purishkevich provided a kind of legitimacy; one can imagine the wrath of the Tsaritsa and the Tsar had this been a plot carried out solely by British agents. It had to look as if Russians had done it and had committed murder for the right reasons. The overt involvement of British intelligence would have produced serious repercussions for the relationship between Russia and Great Britain, something that the British wished to avoid at all costs.

Whoever first conceived the idea of assassinating Rasputin I am unclear about. Was it Scale, Alley and Rayner or Yusupov, Dmitrii and Purishkevich? It is fairly clear though that from the time of Yusupov and Purishkevich's first meeting things moved rapidly towards the demise of "Dark Forces".

There were multiple motives for killing Rasputin but to the British a separate peace between Russia and Germany would have been devastating. Rayner's obituary states that he was present in the palace at the time of the murder. While this is hearsay it was a strongly held view within his family and it was he and no one else who told his family this. We know from Yusupov that he knew of the plot and we know he was with Yusupov the morning following the murder and escorted him to the railway station. Purishkevich had approached Samuel Hoare, the head of the SIS station, about the plot; was this to gain British involvement? I think some of the truth around the events of that night are to be found in Lazovert's account but what comes through so strongly is the attempt by all to convince the reader that only those mentioned in the "accepted version" of events were present.

At some stage Rayner enters the scene. From his relationship with Yusupov, Rayner has developed a hatred towards Rasputin and believes that he knows much about Germany's plans. Together with a not unusually drunken Rasputin, the ingredients are in place for torture and death. Is tort tire a flippant aside? No, a fact that is borne out by the details of the forensic examination. The injuries contained both in Kosorotov's report of the post mortem and in Zharov's reexamination are extensive arid severe. The body's collision with the bridge supports when thrown into the river accounts for some of them, but others were caused while he was alive. Yusupov's account of his frenzied attack on the body after it had been dragged back into the palace, having been shot by Purishkevich, does not hold water, firstly because we know from the forensics that the whole running-away-across-the-yard episode is a massive lie, a total fabrication....

Professor Pounder is not given to making wild claims; his professional integrity depends on assessment of forensic evidence. Nevertheless, he is certain that the wound to Rasputin's head was caused by a .455 Webley, the standard British issue side arm in the First World War. I also concluded from my own assessment that the wound was caused by that type of weapon, while Zharov is clear that it was made by a larger-calibre weapon than the other two used. Variously we are told Dmitrii had a Browning, Yusupov a pocket Browning and Purishkevich a Savage - all automatic weapons and not bulky revolvers.

Who would have used a .455 Webley? The simple answer is Oswald Rayner, the same Rayner who was Yusupov's close friend, the man who had prior knowledge of the plot and would be in Yusupov's presence after the murder. Professor Pounder's evidence became even more compelling as he told me: "The prominent grazing around the margins of the wound, visible in the post mortem photograph, is indicative of the marginal abrasion caused by a non-jacketed - i.e. lead - bullet and in this case one of a large calibre. The use of a lead non-jacketed bullet strongly indicates a revolver rather than a pistol." For those who have sought to disparage the theory, that the British SIS were involved and that it was Rayner who fired the fatal head shot, Pounder's evidence is damning...

It is most unfortunate for us that prior to his death Oswald Rayner destroyed all his files. A guilty conscience, or details of a heinous crime that brought little credit on those involved? We will never know.

The key link between the British secret service bureau in Petrograd and the Russians plotting Rasputin's demise was Rayner through his relationship with Prince Yusupov, the leader of the Russian plotters. Yusupov enticed Rasputin to his family's palace on the banks of the river Neva in Petrograd for a "party", with the prospect of sex apparently high on the agenda. Yusupov told his wife Princess Irina, the Tsar's niece, that she was to be used as "the lure" to entice Rasputin to attend the party, a suggestion that appears to have persuaded her to extend a holiday in the Crimea so she was not in Petrograd at the time. Those known to have been present for the "party" in the Yusupov palace, apart from Rasputin, include Yusupov himself; the Grand Duke Dmitry Pavlovich, the Tsar's second cousin; Purishkevich; Lieutenant Sergei Sukhotin, a friend of Yusupov's; Dr Stanislaus de Lazovert, the medical officer of Purishkevich's military unit, who was recruited as the driver; plus Rayner.

Once there Rasputin was plied with drink and then tortured in order to discover the truth of his alleged links with a German attempt to persuade Russia to leave the war. The torture was carried out with an astonishing level of violence, probably using a heavy rubber cosh - the original autopsy report found that his testicles had been "crushed" flat and there is more than a suspicion that the extent of the damage was fuelled by sexual jealousy. Yusupov, who is believed to have had a homosexual relationship with another of the plotters, the Grand Duke Dimitri, is also alleged to have had a previous sexual liaison with Rasputin. Whatever Rasputin actually told the conspirators, and someone in his predicament could be expected to say anything that might end the ordeal, they had no choice then but to murder him and dispose of the body. He was shot several tunes, with three different weapons, with all the evidence suggesting that Rayner fired the final fatal shot, using his personal Webley revolver. Rasputin's body was then dumped through an ice hole in the Neva.

Yusupov's account details not only his own role in the murder, but also that of Grand Duke Dmitri, Vladimir Purishkevich and Dr Lazovert, as well as Captain Sergei Soukhatin. However, in the days that followed, there were rumours of a sixth conspirator in the palace. Someone else was said to have been present that night - a professional assassin who was working in the shadows.

What Yusupov was at pains to conceal was that Oswald Rayner, a key member of the Russian bureau's secret inner circle, had also been there that night. His critical role in the killing might have remained a secret for all time had it not been for a fatal mistake on the part of the murderers.

The mistake occurred in the aftermath of the murder, when the plotters were disposing of the body. Yusupov and his friends had assumed that the corpse would sink beneath the ice and be flushed out into the Gulf of Finland. There, trapped under the ice for the rest of the winter, it would be lost forever. What they had never expected was that Rasputin's corpse would be found and plucked from the icy waters.

Rasputin's corpse was spotted in the Neva River on the second full day after his death. A river policeman noticed a fur coat lodged beneath the ice and ordered the frozen crust to be broken. The body was carefully prised from its icy sepulchre and taken to the mortuary room of Chesmenskii Hospice. Here, an autopsy was undertaken by Professor Dmitrii Kosorotov.

The professor noted that the corpse was in a terrible state of mutilation: "his left side has a weeping wound, due to some sort of slicing object or a sword. His right eye has come out of its cavity and falls down onto his face... His right ear is hanging down and torn. His neck has a wound from some sort of rope tie. The victim's face and body carry traces of blows given by a supple but hard object: These injuries suggest that Rasputin had been garrotted and repeatedly beaten with a heavy cosh.

Even more horrifying was the damage to his genitals. At some point during the brutal torture, his legs had been wrenched apart and his testicles had been "crushed by the action of a similar object." In fact, they had been flattened and completely destroyed.

Other details gleaned by Professor Kosorotov suggest that Yusupov's melodramatic account of the murder was nothing more than fantasy. Yet it was fantasy with a purpose. It was imperative for Yusupov to depict Rasputin as a demonic, superhuman figure whose malign hold over the tsarina was proving disastrous for Russia. The only way he could escape punishment for the murder was to present himself as the saviour of Russia: the man who had rid the country of an evil force.

The story of the poisoned cakes was almost certainly an invention: the postmortem included an examination of the contents of Rasputin's stomach: "The examination," wrote the professor, "reveals no trace of poison."

Professor Kosorotov also examined the three bullet wounds in Rasputin's body. "The first has penetrated the left side of the chest and has gone through the stomach and liver," he wrote. "The second has entered into the right side of the back and gone through the kidney." Both of these would have inflicted terrible wounds. But the third bullet was the fatal shot. "It hit the victim on the forehead and penetrated into his brain."

It was most unfortunate that Professor Kosorotov's post-mortem was brought to an abrupt halt on the orders of the tsarina. But the professor did have time to photograph the corpse and to inspect the bullet entry wounds. He noted that they "came from different calibre revolvers."

On the night of the murder, Yusupov was in possession of Grand Duke Dmitrii's Browning, while Purishkevich had a Sauvage. Either of these weapons could have caused the wounds to Rasputin's liver and kidney. But the fatal gunshot wound to Rasputin's head was not caused by an automatic weapon: it could only have come from a revolver. Forensic scientists and ballistic experts agree that the grazing around the wound was consistent with that which is left by a lead, non-jacketed bullet fired at point-blank range.

They also agree that the gun was almost certainly a British-made .455 Webley revolver. This was the favourite gun of Oswald Rayner, a close friend of Yusupov since the days when they had both studied at Oxford University.