Edward VIII was secretly bugged during the final days before his abdication on the order of his own ministers, according to official files made public on Thursday.
Papers held secret in the Cabinet Office for more than seven decades show that home secretary Sir John Simon instructed the General Post Office (GPO) covertly to monitor the King's telephone calls.
The action suggest an extraordinary breakdown of trust between Edward and his ministers as he wrestled with his decision as to whether he should give up his throne or the woman he loved.
In December 1936, the crisis was coming to a head. The previous month the new King, who had not yet been crowned, informed prime minister Stanley Baldwin that he intended to marry his mistress, American divorcee Wallis Simpson.
At a time when the Church of England refused to remarry divorcees while their former spouse was still living, Baldwin was adamant that such a match was impossible if Edward was to remain on the throne.
After months of silence the story had finally broken in the press - prompted by a sermon by the Bishop of Bradford on the King's duties as head of the Church of England.
At first it appeared that Edward believed that he could defy his ministers and ride out the crisis on a wave of popular support.
There were suggestions he could go ahead with the marriage to Mrs Simpson - forcing Baldwin to resign - and then appoint a new "royalist" government headed by Winston Churchill.
It was against this backdrop - with the King holed up at his residence at Fort Belvedere in Windsor Great Park and Mrs Simpson in the South of France where she had take refuge with friends - that Simon instructed the GPO to start intercepting his calls.
The files released by the National Archives in Kew, west London, include the draft of note - dated December 5 1936 and marked "Most Secret" - sent from the Home Office to the head of the GPO Sir Thomas Gardiner confirming Simon's order.
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.