Formerly secret MI5 files have revealed that the plans for the D-Day landings in 1944 were leaked to one of Britain's brightest military strategists.
Sir Basil had the D-Day plans three months before the landings
Sir Basil Liddell Hart had all the details three months before the invasion took place.
Some of the once-classified files have now been given to the National Archives in Kew, west London.
They suggest the situation may have endangered the assault, which was a major turning point in World War II.
Sir Basil, who was decorated for bravery in World War I, began publishing his military theories during the 1920s and is also credited with greatly influencing the development of armoured warfare.
He was close to many top generals throughout World War II.
MI5's files reveal that not only were the details given to Sir Basil, but that he also began boasting about them around London.
He even prepared a critique of the plan, entitled Some Reflections on the Problems of Invading the Continent, which he circulated among leading politicians and military figures.
When Prime Minister Winston Churchill was informed he was outraged and demanded that Sir Basil be prosecuted.
Sir Basil, however, insisted that he had worked out the plan for himself and that as a military expert he was entitled to be consulted on such matters.
Despite Churchill's anger, no legal action was ever taken, although Sir Basil was placed under surveillance and his telephone calls and letters were intercepted.
One MI5 report complained at the time that he was an "unconscious ally" of Josef Goebbels, the Nazi propaganda chief who regularly quoted him in his radio broadcasts.
However, there was no suggestion that Sir Basil was involved in any subversive activity.
'Very considerable risk'
Howard Davies, from the National Archives, said the leak had created a situation which was taken seriously by the nation's intelligence services.
"I think the files being released show there was a very considerable risk," he said.
"We now know for the first time that the fact that Basil Liddell Hart was a well-known military tactian would have lent weight to any information he did, whether deliberately or accidentally, pass on.
"MI5 were understandably very concerned that he knew these plans."
MI5 narrowed down the most likely source of the leak to General Sir Tim Pile who was in command of anti-aircraft defences, but could not find enough proof to take further action.