Questions about this extraordinary leak will be among those that the Home Secretary, Jack Straw, will want answered. Who initiated the contact, when, and, perhaps most of all, why? The key question is whether Mitrokhin sought out Professor Christopher Andrew, or whether MI5 encouraged the marriage of mind and KGB information - a scenario that appears far more likely. Either way, his involvement speaks volumes for the relationships he has built up with figures in the world of espionage.
Professor Andrew would make a perfect spy himself. The tall, enigmatic Cambridge don is renown for being the soul of discretion and for keeping his own research projects safely under cover. The historian has long been the most authoritative voice on British intelligence and deeply trusted by the security services. So perhaps it comes as no surprise that he should end up as guardian to the goldmine of KGB secrets smuggled out of Russia by Colonel Vasili Mitrokhin.
Before his defection, Mitrokhin had enjoyed unique access to the KGB's most secret archive, containing files on all its agents and operations from 1917. Defection to a better life in the West was on the Colonel's mind long before he retired in 1984. He had systematically copied thousands of documents and buried them in waterproof trunks.
Soon after the Cold War ended, he approached CIA officials and offered to defect. He was turned down - the West was already besieged by aspiring defectors and the CIA did not think he was useful.
MI6 took a different view when he approached its officers in Latvia. In December 1992, Mitrokhin, his family and six large trunks of files were smuggled out of Russia into Britain. MI6 handed over Mitrokhin and his archive to MI5 who would have debriefed him. An analysis of the material would have been made. It was potentially a treasure trove, capable of demonstrating the strengths and weaknesses of Western intelligence and counter intelligence over 60 years.
But for some inexplicable reason, KGB agent "Hola", Melita Norwood was not identified.In the mid-1990s, British intelligence decided that its "coup" should be made public and introduced Mitrokhin to Professor Andrew, the level-headed academic relied upon as a pair of "safe hands".
MI5's failure to pass a summary of the Mitrokhin archive to the Conservative Home Secretary suggests it did not grasp the impact the material would have if made public.
But Professor Andrew did. He must have felt he had won the lottery. The files contain information that will have an impact on every Western country. A fellow expert said yesterday: "Mitrokhin could have approached him independently. But the most likely scenario is that such a person would have sought permission from MI5 or SIS before collaborating. There is a strong tradition in the British Secret Services of anxiety to manage information. In the past MI5 and SIS have encouraged publications to be guided and have put people together with trusted people."
For years the 58-year-old professor diligently analysed the explosive archives without a whisper of a leak. A fellow academic said: "He enjoys keeping his own projects under cover. It is a bit of a joke between us."
Despite brief visiting professorships abroad, the professor of modern and contemporary history has rarely strayed far from Cambridge. He arrived at Corpus Christi as a young undergraduate in the early Sixties, when he married Jennifer Garratt. The couple have a son and two daughters.
After a history double first and PhD on pre-1914 French foreign policy, he gained a research fellowships at Gonville and Caius alongside Stephen Hawking. In 1985, he wrote Secret Service: The Making of the British Intelligence Community and in 1990 he co-wrote the first of the Gordievsky books about the KGB.
Yesterday some insiders insisted that the choice of Professor Andrew was symptomatic of the disregard for freedom of information that still abounds in the secret services. Fellow experts said the archives should have been placed within the public record office for all to see.