The Secret Royals: Spying and the Crown, from Victoria to Diana by Richard J. Aldrich and Rory Cormac

Princess Diana summoned a butler one day to help to shift furniture in her sitting room at Kensington Palace. They rolled up the carpets and underlay and took up the floorboards.

Diana was searching for hidden microphones, but found nothing. “She worried about devices being placed in plug sockets, light switches or lamps,” Paul Burrell, the butler in question, later revealed. She was so convinced that “dark forces” were watching her that she even took down a mirror, after being told that conversation could bounce off the glass and be monitored outside.

Was she under surveillance by MI5? The “Squidgygate” tapes of 1992 — conversations between Diana and James Gilbey, a childhood friend with whom she denied any affair — were supposedly overheard by a retired bank manager with a radio scanner. But security experts suggested it was more likely that Diana’s telephone was tapped.

The home secretary Ken Clarke dismissed that idea as “extremely silly”, but she wouldn’t have been the first member of the royal family to come under the scrutiny of the security services.

The Secret Royals, written by a professor of international relations and a professor of international security, records the close links between the royal family and the world of espionage and security. Some were avid consumers of secret reports, while others were under close watch.

Prince Philip, According to M.I.5

Prince Philip could have told Diana a thing or two about surveillance. George VI asked Special Branch to vet his potential son-in-law. There were doubts because, although he had a good war record, his family had Nazi connections (the Queen Mother is said to have referred to him as the Hun). MI5 reported that his rooms were messy, his language “coarse” and that he enjoyed late-night drinking.

Edward VIII seemed to be under almost constant surveillance. George V so despaired of his heir that he had Edward watched by Special Branch, which it is claimed opened Wallis Simpson’s letters and tapped her phone.

The monitoring continued when Edward became king. Late one December night in 1936, a young MI5 officer called Thomas Robertson stealthily entered Green Park and placed a wiretap inside the telephone junction box that served Buckingham Palace. From here he and his colleagues — acting on the orders of the prime minister, Stanley Baldwin — listened to conversations between the King and his brother. Robertson was the first outsider to hear news of Edward’s abdication.

When Edward was in exile in Paris, his trusted private detective sent reports back to London. The surveillance was stepped up during the war, when he and his wife, Wallis, were seen as a high security risk. As a foreign office report put it: “The Duke of Windsor is notoriously pro-Nazi. He is also a heavy drinker, and what few wits he had have wilted.”

The Germans thought he would be an influential voice calling for a peace deal, and could even be a puppet sovereign-in-exile. A Nazi agent made a comic attempt to prevent the Windsors leaving Lisbon for Edward to become governor of Bermuda; a car containing the couple’s clothes was sabotaged in the hope that they wouldn’t depart without their extensive wardrobe.

The book, which stretches back to Elizabeth I and her spymaster Sir Francis Walsingham, has something of interest on pretty much every page. Edward was apparently so careless with sensitive papers that he once handed his red boxes to the American air attaché at his retreat near Windsor and asked him to take them to Buckingham Palace.

Perhaps the monarch who was most enthusiastic about the world of intelligence was Victoria, who was instructed in the dark arts — reading and resealing letters, sending misinformation to foreign powers — by her uncle, King Leopold of the Belgians.

“The Duke of Windsor is notoriously pro-Nazi. He is also a heavy drinker, and what few wits he had have wilted.”

Her government had no formal intelligence operation: Special Branch was created in response to Irish terror attacks in the 1880s, but there was no official secret service until the creation of the war office’s special duties section in 1909. So Victoria became an early version of Judi Dench’s M, running her own network of agents across Europe. “Queen Victoria selected the most intelligent member of each European royal family, and ‘on any question … she obtained an opinion’.”

The most valuable agent in the field was her daughter Vicky, who was married to the German crown prince. She sent crates of sensitive documents to England from the Prussian court, and communicated with her mother by cipher to avoid the attentions of Bismarck’s counterintelligence efforts. She took huge risks, and her role was so sensitive that, after the Second World War, Anthony Blunt visited her ancestral home in Germany and smuggled Vicky’s personal papers back to Windsor.

Blunt, then surveyor of the king’s pictures, had worked for MI5 during the war, but since 1937 had also been working for the Russians. “The royal family knew that Blunt was a KGB agent as early as 1948,” the authors say. Blunt was under constant surveillance, but it wasn’t until 1964 that he formally confessed in return for immunity from prosecution.

Queen Victoria became an early version of Judi Dench’s M, running her own network of agents across Europe.

The Queen usually takes security and intelligence matters very seriously, but not always. She is reported to have joked about “putting something in the coffee” of a troublesome Jordanian, and later suggested that she would like to hit the Ugandan dictator Idi Amin over the head with a ceremonial sword.

And she has taken part in shadowy diplomacy without even knowing it. In 1953 it was feared that the Shah of Iran was about to abdicate, handing power to his anti-British prime minister. In the middle of the crisis the shah learnt of an encoded telegram “from Anthony Eden from Queen Elizabeth” urging him to stay.

This support from a fellow monarch stiffened the shah’s resolve, but there was just one problem. The message was indeed from Queen Elizabeth, but not that one. Eden sent his telegram while travelling to Canada, aboard the liner of the same name.

Roland White is a London-based journalist and a former Atticus columnist for The Sunday Times