Captain Basil Liddell Hart, a historian and one of Britain's most distinguished military strategists, discovered the details of the plan - codenamed "Operation Overlord" - and began boasting about them around London.
In files released by Britain's domestic intelligence service, MI5, to the National Archives, it was also revealed that Liddell Hart had prepared a critique of the plan which he circulated among some of the country's politicians and senior military officials.
On what has come to be known as D-Day, more than 100,000 Allied soldiers, made up of contingents from the United States, Britain, Canada, France and Poland, landed on the beaches of Normandy, northwestern France.
Many historians regard it as the turning point of World War II. It led to the capture of Paris about two months later, and the eventual fall of Nazi Germany the following year.
'D-Day doubts'
MI5 first learned that Captain Liddell Hart, who had been invalided out of the army after being gassed in World War I, knew of Overlord when he mentioned to Duncan Sandys, then a minister in the Ministry of Supply, that he had "strong doubts" about the plans while the two were having lunch on March 10, 1944.
At the time, the plan was supposedly known only to a handful of senior Allied figures.
Mr Sandys declined to comment on the historian's statement, but Captain Liddell Hart pressed on, giving the minister a copy of his paper critiquing Overlord.
Mr Sandys later reported his conversation to General Hastings Ismay, military adviser to the British wartime prime minister Winston Churchill – General Ismay quickly passed the information along to Churchill.
When Mr Churchill got wind of Captain Liddell Hart's actions, he was outraged and demanded that the historian be prosecuted.
It also emerged that the strategist had given a copy of his critique to Labour Party politician Stafford Cripps, Lord Beaverbrook, a former minister for supply under Churchill, and three American generals.
Captain Liddell Hart was subjected to two hours of questioning at the hands of General Ismay and another military official. He claimed that he had come to know about the operation purely through his own inferences.
MI5 refused to believe Captain Liddell Hart's assertion that he hadn't been told of the plan, however, and narrowed down the most likely source of the leak to General Tim Pile, then in charge of anti-aircraft defences.
In the end, no action was taken against the strategist, or General Pile, though Captain Liddell Hart was placed under surveillance and all his telephone calls and letters were intercepted.
Other documents handed over by MI5 to the National Archives also contain evidence of the author Graham Greene's activities as an operative for the British foreign intelligence service MI6.
In 1943, Greene, whose books include "Brighton Rock" and "The Heart of the Matter", was trying to track down a fireman who he suspected was an MI5 double-agent code-named Rhubarb.