The leaking of one of Britain's greatest state secrets was foiled when a Sunday Express journalist's notebook was left on the floor of the Old Bell pub in Fleet Street.
The notebook contained never before seen details of Britain's top secret code-breaking site Eastcote, which was later to become the Government Communications Headquarters (GCHQ).
Sunday Express reporter Eric Tullett had been passed the explosive information detailing Britain's operation to intercept and decode Soviet signals by Arthur Askew, the Foreign Officer's former head of physical security.
But his extraordinary scoop was lost when he left his notebook on floor of the Old Bell pub, on London's Fleet Street.
At the time the public were in the dark about the Cold War cipher work being carried out at Eastcote. Nor did they know about Bletchley Park, the wartime cryptography site which pioneered the art of using early computer technology to break encrypted messages.
The secret survived for another 23 years until the existence of GCHQ was finally revealed in the mid 1970s.
At some point, Askew had told Tullett of the code cracking operations going on at GCHQ which had been transferred from Bletchley Park after the Second World War.
His motive for the betrayal of Britain's intelligence services is not clear, but it is thought he was furious at newspaper reports that he was to blame for the escape of Guy Burgess and Donald Macclean, part of the Cambridge Five spy ring.
The pair escaped to Moscow in 1951 when they came under suspicion of spying for the Soviets.
Soon after Askew retired, but media reports suggested he had resigned as part of a reorganisation of Britain's security services after the embarrassing incident.
Police records indicate that a man, assumed to be Tullett, rang them inquiring as to whether a notebook in a leather case had been handed in as found property. But instead a barmaid had found the notebook and, noticing that its pages contained words such as "Cipher Machines", "Moscow" and "Secret", alerted the police.
The police handed the book to MI5 who then returned it to Tullett. The Foreign Office allowed part of Tullett's story to go ahead - but the explosive references to GCHQ and its secret work were redacted.
The Foreign Office's new head of security, George Carey-Foster, wrote in a letter to a senior MI5 official Dick White: "I think the only thing we can do is to redraft these articles and hope for the best."
Historian Christopher Moran from Warwick University told the Radio 4's Today Programme: “Buried within Tullett’s notebook is a humdinger of a secret. The existence of GCHQ was not revealed to the public until the early to mid 1970s, yet Tullett has it in his notebook in July 1951.
“It is only today in 2012 that we can actually discover just how close Tullett and Askew got to revealing what was without question the greatest British post war secret."
Askew's secret may have stayed hidden from the public, thanks to the sharp-eyed barmaid, but 60 years on many former intelligence officials cannot forgive him.
Jean Valentine, who worked at Bletchley park during the war said: "He should not have said a word. If all we thousands of people could keep quiet that long, this one arrogant civil servant cannot just open his mouth and chatter. He should have been shot."