Ministers ordered the bugging of Edward VIII's telephones in Buckingham Palace and in his Windsor retreat at the height of the 1936 abdication crisis.
The move is disclosed in a cache of intelligence files hidden until now in a basement at the Cabinet Office in central London.
Among them is a note, dated December 5, 1936, and marked "most secret", from the Home Office to the head of the General Post Office, Sir Thomas Gardiner, referring to an order from home secretary John Simon.
It states: "The home secretary asks me to confirm ... that you will arrange for the interception of telephone communications between Fort Belvedere and Buckingham Palace … and the continent of Europe."
When not at the palace, Edward stayed at Fort Belvedere, his bolthole in Windsor Great Park. Edward's mistress, the American divorcee Wallis Simpson, was with friends in southern France at the time.
The panic in the British establishment provoked by Edward's affair with Simpson and his belief he could get away with marrying her and remain king has been widely reported.
What has not been disclosed until now is how the lack of trust in the monarch was such that ministers recorded his personal conversations.
The government's anxiety led to a close watch of outgoing telegrams. One that was intercepted and blocked was from Neil Forbes Grant, London editor of the Cape Times.
Summoned to see the home secretary, Grant was told there was no truth to his report that the king was about to abdicate and that if the news had reached South Africa and then been telegraphed back to Britain, the reaction might have been "of a most serious character".
Grant insisted he had his information from "a very highly placed source", but seemed suitably chastened.
Edward abdicated on December 10, 1936, four days after Grant sent his intercepted telegram.