Interesting Timing, Ivan

Hitler had just been defeated, Stalin was victorious and Viktor Bogomolets was down on his luck. After more than three decades spying for British intelligence, Bogomolets, who began working for MI6 shortly after the Russian revolution, was curtly informed that he had been stripped of his British citizenship.

It was at this point that Bogomolets decided to betray his British masters. According to papers declassified yesterday by Russia's foreign intelligence service, the SVR, he became one of Moscow's most accomplished double agents.

In 1945 he began spying for the Soviet Union, passing crucial information back to Moscow about British intelligence at the height of the Cold war. Codenamed "Britt" by his Soviet handlers, Bogomolets' reports were circulated among the top echelons of the Soviet Union's leadership - and were even read by Stalin himself.

The double agent also betrayed the man who had recruited him to MI6 in the first place, Colonel Harold Gibson. Gibson was responsible for a network of undercover British agents working deep inside the Soviet Union. Soviet intelligence tracked Gibson closely until his death in 1960, the documents show.

Major General Lev Sotskov, a former intelligence officer who has written a book based on the new archive material, yesterday described Bogomolets as a "very big fish". The only reason the Russian emigre had not been identified before was that neither the British nor the Soviets had any incentive to unmask him, he said.

"Bogomolets was extremely important. He wrote a 100-page memo detailing all his contacts in British intelligence soon after defecting. He was involved in at least two major secret intelligence operations. What we know about him is the tip of the iceberg," Gen Sotskov said.

The revelation suggests that in an era already famous for its treachery and double dealing, MI6 was even more compromised than previously thought.