A CELEBRATED military writer and strategist was investigated by MI5 during the Second World War because he appeared to have acquired one of the biggest secrets — details of the Normandy landings, four months before they took place.

Captain Basil Liddell Hart, Britain’s leading strategist on tank warfare, who was later knighted, shook the Government and military establishment when they discovered that he even knew the names of the beaches on which thousands of Allied troops were due to land on June 6, 1944.

He delivered his bombshell in the form of a report that he showed to Duncan Sandys, then a junior minister in Winston Churchill’s Government, in March 1944. Other copies were sent to Lord Beaverbrook, then Lord Privy Seal, Sir Stafford Cripps, then Minister of Aircraft Production, and three American generals.

Sandys, who was Churchill’s son-in-law, was not one of the few people in the Government who knew about Operation Overlord but as parliamentary secretary in the Ministry of Supply he had some knowledge of what was brewing and tipped off General Hastings Ismay, deputy secretary to the War Cabinet, about the report.

The repercussions of what appeared to be a serious security breach that could have had fatal consequences had the Germans learnt of the contents of the report are revealed in the files released by the National Archives.

Churchill had to be told. So was General Dwight Eisenhower, Supreme Commander of the Allied Expeditionary Force. MI5 was brought in to try to find out who might have revealed the secrets of Operation Overlord to Liddell Hart.

The investigation came as a surprise to Christopher Andrew, the Cambridge University professor who is the official historian of MI5.

He said: “The most important Allied secret was where the D-Day landings were going to take place. Liddell Hart produced a paper that identified the beaches. I had not the slightest inkling this file existed.”

The military writer was interviewed by General Ismay and a Brigadier Jacob, but Liddell Hart denied he had been given any secrets. He insisted he had worked it all out for himself.

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