'It seems pretty clear the pair of them have gone off'

It is 29 May 1951. Two senior figures in the British diplomatic service – Guy Burgess and Donald Maclean – have disappeared and are beating a secret retreat, we now know, to Moscow, wanted by their British spymasters. Soon it will be all over the newspapers, the first big bang from which exploded the greatest scandal in the history of international espionage.

But the story, as told from inside MI5, by its affable deputy director Guy Liddell, now makes for highly ironic reading. His diaries of the immediate post-war period, which have been revealed this morning by the national archives, show not only that he didn't have much of a clue about what was going on, but what scant detail he did have he shared with the other members of the now notorious "Cambridge Four", Kim Philby and Anthony Blunt, pictured below.

"The Watchers [specially trained surveillance policemen] failed to pick up Maclean since his departure for the country on Friday, and we now learn from the Foreign Office that he was given a day's leave on Saturday," records Liddell in his diary.

He then receives a phone call from an individual whose name has been removed, concerning Mr Burgess's mysterious disappearance.

"He had not been seen since Friday and had telephoned either on Thursday or Friday to the wife of Goronway Rees [an academic friend], when he said, amongst other things, that he was not likely to see her for a long time. Burgess had remarked that he was going away for the weekend in order to assist a friend who was in some sex trouble and being blackmailed," he states rather simply, before deciding it is, "likely to be untrue.In view of past association between Burgess and Maclean observed by Watchers, it seems pretty clear that the pair of them have gone off".

The two men were both gay and had been students at Cambridge.

Instructions were sent to all border points to detain the men, but the pair remained hidden for five years, emerging in Moscow in 1956. With the world's press clamouring for information, and questions being asked of Liddell by everyone from Malcolm Muggeridge to the Prime Minister and the King, the conclusions of Liddell's questioning of the men's associates now look rather unfortunate.

"It seemed to me unlikely that a man of Burgess's intelligence could imagine that he had any future in Russia," he wrote. "I find it difficult to imagine Burgess as a Comintern agent or an espionage agent... I feel certain that Anthony [Blunt] was never a conscious collaborator with Burgess in any activities."

It was the disappearance of the two men that would come to unmask Kim Philby, the most senior spy in the group who sent so many confidential documents to the Russians that they were sometimes unable to translate them all. Were it not for Burgess, he remarked years later, he might have made it to the top job at MI6.

Two months later Philby resigned from his position at MI5 and he was interrogated later in the year by Helenus "Buster" Milmo QC. Milmo, wrote Liddell, was "firmly of the opinion that he is or has been a Russian agent, and that he was responsible for the leakage about Maclean and Burgess. I feel less convinced about this last point. Philby's attitude throughout was quite extraordinary; he certainly did not behave like an innocent person."