Edward VIII was bugged by ministers before abdication

It stated: "The Home Secretary asks me to confirm the information conveyed to you orally, with his authority by, by Sir Horace Wilson that you will arrange for the interception of telephone communications between Fort Belvedere and Buckingham Palace on the one hand and the Continent of Europe on the other."

The file does not contain any record of the intercepted calls or any other details of the monitoring operation. Five days later, on December 10, Edward finally accepted that he could not prevail and he signed the instruments of abdication. His reign had lasted just 326 days.

The files do however show just how sensitive ministers were during those febrile December days. On December 6, a Home Office official reported that he was rung at home at 11pm by a GPO clerk to report two telegrams sent by a South African journalist claiming the King had already resigned.

On Home Office instructions the telegrams were blocked and the journalist, Neil Forbes, the London editor of the Cape Times, summoned for a dressing down by the Home Secretary.

Simon told Forbes there was "no truth" in his report and said that if the news had reached South Africa and then been telegraphed back to Britain, the reactions might have been "of a most serious character".

"I reminded him that in 1815 a false rumour that we had lost the Battle of Waterloo produced a financial crisis and ruined many people," Simon wrote.

"I asked him if he did not realise that his responsibilities as a journalist and an Englishman made the sending of such a message without definite authority as to its truth very improper and reckless."

Grant insisted that he had received his information from "a very highly placed source" and was convinced that "the decision had been taken and would be officially announced within the hour".

One enterprising Indian journalist did get round the censors and succeeded in filing a report on December 7 that the instruments of abdication had been signed - three days before it actually took place.

A cable was sent from Calcutta attempting to alert the British press to the "news", but was not apparently followed up.

Gill Bennett, the former official historian at the Foreign Office who reviewed the files for release, said they were among a large number of papers collected by successive cabinet secretaries down the years which had been regarded as "too difficult" to know what to do with.