The man investigating the defection of two of the notorious "Cambridge spies" was confiding in members of the same group of Soviet double agents unknowingly, records have revealed.
The personal diaries of Guy Liddell, deputy director general of MI5, describe the moment security services realised Guy Burgess and Donald Maclean had fled to the Soviet Union in May 1951.
And as he gathered information, Liddell had no idea he was sharing it with other members of the high-level Soviet espionage ring.
(clockwise from left) Anthony Blunt, Guy Burgess, Donald MacLean and Kim Philby who tipped off Burgess and MacLean in 1951 forcing them to defect and then defecting himself in 1963.
Burgess and Maclean fled Britain on May 25 1951 after a warning by fellow double agent Kim Philby - who was working for MI6 in Washington - that Maclean was about to be unmasked.
The trio had been recruited by Soviet intelligence in the 1930s while they were Cambridge undergraduates.
Between them they were responsible for passing thousands of top-secret documents to the Russians.
Liddell's post-war diaries document the run-up to Maclean and Burgess' disappearance, as well as the aftermath in which suspicion fell on Philby and later on fellow MI5 employee Blunt - the so-called "fourth man" in the spy ring.
The diaries, released to the National Archives in Kew, west London, tell how suspicion fell on Maclean in April 1951 but as plans were made to interrogate him he disappeared along with Burgess, whom Philby had sent from Washington to England to warn Maclean.
Liddell wrote on May 29 of receiving a phone call asking if he had "heard about Guy Burgess", who had been sent home from Washington, where he worked with Philby and lived in his house, after getting three speeding tickets in one day.
"It seems pretty clear that the pair of them have gone off," Liddell wrote as their disappearance emerged.
On May 30 Liddell met MI5 agent Tommy Harris and Blunt, who knew Maclean at university and - unbeknown to Liddell - was also working for the Soviets.
Apparently, on a visit to Harris and his wife, when asked about Philby, Burgess put his hands to his head, saying: "Don't speak to me of Kim - nobody could have been more wonderful to me", then burst into tears.
"There may possibly be some significance in this if, in spite of everything the Philbys had done to keep him straight, he had betrayed Kim through getting to know something about the Maclean case and acting on the information," Liddell wrote.
The MI5 boss did not originally believe Burgess was a spy, saying: "It seemed to me unlikely that a man of Burgess's intelligence could imagine that he had any future in Russia."
On July 7, Liddell also recorded there was to be a "highly confidential enquiry in the Foreign Office about the security risks of employing homosexuals". Both Maclean and Burgess were known to be gay.
Suspicion soon moved to Philby, with demands from Washington for him to be interrogated.
But Liddell seemed unconvinced, writing on August 20: "I am still rather inclined to think that it was not a leakage which caused Maclean and Burgess to make their hurried departure."
MI5 interrogator "Buster" Milmo, who questioned Philby on December 12 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", Liddell recorded.
Personally I feel less convinced about this last point. Philby's attitude throughout was quite extraordinary; he certainly did not behave like an innocent person.
"He never made any violent protestation of innocence, nor did he make any attempt to prove his case."
In 1952 Liddell was told that information suggested Blunt was a more active Communist than originally known, but in July he wrote: "While I believe that Blunt dabbled in Communism, I still think it unlikely that he ever became a member".
Again, in that view, he was proved wrong, as Blunt confessed to MI5 in 1964.
Stephen Twigge, head of the Modern, Domestic, Diplomatic and Colonial team at The National Archives, said the diaries revealed how many people around Liddell turned out to be Soviet spies.
"What is interesting is this closed world where everybody knows everyone else," he said.
"Ironically, most of the people he was talking to about Maclean and Burgess' defection were in fact people that would go on to defect."