The security services listened in on the phone calls of King Edward VIII at the height of the abdication crisis as Whitehall desperately sought to prevent news of his imminent departure from leaking out, according to previously unseen documents.

The extraordinary revelation that MI5 was asked by the Home Secretary in 1936 to intercept all phone calls between the King’s country residence, Buckingham Palace and London addresses likely to include the home of Wallis Simpson underlines the sense of panic gripping Britain’s governing elite at the height of the constitutional meltdown.

The top secret decision by the executive to eavesdrop on the monarch also included a request to monitor calls made from Edward VIII’s stately home, Fort Belvedere, to “Continental Europe”, a likely reference to the south of France where Mrs Simpson was staying at the height of the crisis in December 1936.

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The previously unknown royal manoeuvrings by the Security Service, which are detailed in a handwritten note on behalf of then Home Secretary Sir John Simon, are revealed in documents released today by the National Archives in Kew, west London.

The material is contained in a remarkable cache of documents kept in a locked safe room in the Cabinet Office detailing the difficult decisions faced by successive Cabinet Secretaries, the most senior civil service post, between 1936 and 1951.

Although there were almost constant consultations between Buckingham Palace and Downing Street as the crisis sparked by Edward VIII’s insistence on marrying the American divorcée came to a head, the documents show that the government of the day, led by Conservative prime minister Stanley Baldwin, went to extreme lengths to ensure it knew what the Royal Family was doing.

A memo to MI5 confirming the phone tapping request on 5 December 1936 – five days before the abdication – said: “The Home Secretary asks me to confirm the information conveyed to you orally with his authority that you will arrange for the interception of telephone communications between Fort Belvedere and Buckingham Palace to certain addresses in London on the one hand and the Continent of Europe on the other hand.”

The phone tapping was part of a concerted effort by the government of the day to maintain the tightest possible control of events as Britain lurched into the greatest constitutional crisis of the last century.

The documents show the operation even extended to thwarting a remarkable scoop by a South African newspaper by intercepting a telegram from its London correspondent announcing the abdication four days before it happened.

Neil Forbes Grant, the UK editor of a group of South African papers including the Cape Times, must have thought he had got the story of his life when he sent this cable to his editors in Johannesburg on 6 December: “King has abdicated leaves England tomorrow. Stop. Arrangement may be possibly altered but that is agreement reached tonight – Grant.”

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But the documents show that Mr Grant had not reckoned with the formidable surveillance powers of the British state which meant that Post Office alerted the Home Office to any telegrams being sent out of Britain which might have threatened national security, resulting in the interception of his cable.

In a remarkable exchange which provides insight into the somewhat patrician views of press freedom in 1930s Britain, Mr Grant was personally summoned by the Home Secretary and delivered a dressing down for his “un-English” conduct in breaking news which had not been publicly confirmed by the authorities.

In a first-hand account of his 15-minute “interview” with the wayward journalist, Sir John Simon wrote: “I told him that the message had been brought to my attention before despatch and that I had stopped it. There was no truth in the statement it contained and if it had reached South Africa and had been telegraphed back here the reactions might have been of a serious character.”

Underlining the gravity with which the government viewed the matter, the minister added: “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.

“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 definitive authority as to its truth improper and reckless.”

According to the memo, a suitably chastened Mr Grant, who later gave up journalism to become a novelist, declined to reveal the name of his “very highly-placed source” but conceded the episode had been “a lesson to him”.

Sir John added: “I avoided saying anything which could be regarded as either lecturing or threatening, though I made it plain to Mr Grant how gravely I regarded the matter.”

Further telegrams in the file show that despite Mr Grant’s reprimand, news of the abdication also appeared in other papers in South Africa and India, forcing strenuous denials from London.

The documents also provide an insight into the efforts of the state to minimise press scrutiny of Mrs Simpson while in her London home next to Regents Park as the crisis began to gather pace in November 1936.

A memo submitted to the Cabinet Office details how Edward VIII approached his Scotland Yard bodyguard to ensure that discrete extra patrols were put in place around the property to deter unwanted reporters and photographers as well as ensure that an adjacent empty house, owned by a gentleman from Baghdad, posed no threat to her security.

When the Daily Mirror published a brief story on 27 November 1936 correctly reporting that additional security had been placed around Mrs Simpson, it seems the government machine for quashing unwanted reporting once more ground into action.

The following day the paper carried a fulsome correction: “We regret having given publicity in our later editions to these reports, for which there is no foundation.”

Double agent: The two faces of Lt Col Dudley Clarke

Attired in a floral dress with a white handbag and a turban, the British intelligence officer would have made a striking sight on the streets of wartime Madrid. All the more so since “she” was one Lieutenant Colonel Dudley Clarke.

The senior MI6 officer, posing as a correspondent for The Times, was arrested in October 1941 after he ventured out onto the streets of the Spanish capital dressed – “down to a brassiere” – as a woman.

Documents released today at the National Archives provide full details for the first time of the incident, which created a panic in London because Clarke, the head of a British deception unit in the Middle East, was en route to Egypt with secret information.

A “top secret” memo sent to MI6 and the Foreign Office detailed how Clarke had told Spanish police he was a novelist who “wanted to study the reactions of men to women in the streets”. The British consul found Clarke to be “calm and unconcerned”, while offering an alternative explanation that he was bringing the clothes to Gibraltar for a friend and had tried them on as a prank. But the memo noted: “This hardly squares with the fact that the garments and shoes fitted him.”

In London, officials underlined that Clarke’s military status was not to be disclosed to the Spanish.

In the end, he was released and allowed to continue his journey to Cairo – and went on to a distinguished military career.

