A scheming princess who should definitely NOT be confused with more recent residents 

No sooner had the scheming princess taken up residence at Frogmore Cottage with her frail and wishy-washy husband than she embarked on a campaign of treachery that baffled the British establishment for years.

After originally being welcomed and given the home of grace and favor in the shadow of Windsor Castle, the pair, who were well-known figures in glossy magazines, embarked on a path of betrayal.

While the man was described as a “charming but almost entirely mindless buffoon”, the most vicious criticism was directed at his “cruel and unscrupulous” wife who harbored “anti-British views”.

It should be quickly emphasized that these residents of Frogmore Cottage are not to be confused with more recent residents of the five-bed property in Windsor Great Park.

This story is about a 20th century predecessors of Prince Harry and Meghan: Princess Marina ‘Mira’ Dmitri of Russia and her husband Prince Dmitri Alexandrovich, refugees of the Russian Revolution, who received a warm hug from their British cousins.

So hot, in fact, that the Mata Hari-esque princess began an affair with the late Queen Elizabeth’s uncle, the Duke of Kent, and gave birth to his child, according to a new book.

The author, Susan Ronald, describes the princess’s espionage activities as follows: “She was always after money and sang her own song for the Italians – and also for the Germans.”

Using a discovered MI5 file, The Mail on Sunday is able to tell this remarkable story of bloody betrayal and deceit in more detail.

From the moment she married her poodle-like Russian prince in 1931, Princess Mira was over the moon — and looking for extramarital sex.

The couple had obtained the lease of Frogmore Cottage through the mother of the Grand Duchess Dmitri (whose cousin was George V). The newlywed couple arrived in Britain in January 1932, and the princess was looking for trouble from the start.

That autumn she became pregnant, and after she gave birth, the Duke of Kent paid the child’s school fees until his death in 1942, when his widow abruptly cut off the money.

Princess Mira’s great friend was fashion designer Anna Wolkoff, who at the start of World War II had joined a secret fascist society in London whose purpose was to help the Nazis overthrow the British government.

Wolkoff was eventually discovered by secret agents, arrested, and sentenced to ten years in prison in 1940.

But the spying had been going on for years. In 1936, the two women left behind their glitzy London lives to take part in a riotous upper-class house party near Salzburg in Austria.

The hosts were English society beauties Lady Duff-Assheton-Smith and Lady Alexandra Haig, who had reportedly moved to Austria to celebrate their lesbian fling.

A wealthy Old Harrovian stockbroker named Thomas Willes stepped into this house of pleasure. The 25-year-old and Princess Mira began a passionate affair.

Four years later, while being questioned by MI5 spymaster Maxwell Knight, Willes recalled meeting the princess and said she was under Wolkoff’s control and was spying for a foreign government.

Willes — whose granddaughter Catherine FitzGerald is married to actor Dominic West (who plays Prince Charles in The Crown) — decided not to see Mira anymore.

But later, back in London, he ran into her at The Savoy, where she told him she had an urgent message to deliver to an Italian. At the time, Mussolini’s fascist government was trying to build its own empire.

Willes told spymaster Knight all this when Wolkoff was on trial.

It later transpired that Mira had accompanied Wolkoff on other trips – to Germany and Hungary in 1937 and 1938, when Wolkoff secretly met notorious Nazi collaborator Lord Haw-Haw and army officer Norman Baillie-Stewart, who had been court-martialed for the providing information to the German intelligence service. But was Mira involved in these cloak-and-dagger adventures?

The MI5 file on possible royal family spies shows that the couple’s London flat has been raided to make it look like a burglary. Every letter addressed to them was steamed open and read. Phones were tapped and Mira’s address book was secretly copied.

It was discovered that the Prince’s mother, Grand Duchess Xenia, had been loaned Craigowan Lodge on the Balmoral estate. Was she a spy too?

The Grand Duchess was known to support the Germans’ determination to rid Russia of the communists. Was she sending messages to the enemy from Balmoral, the heart of the royal enclave?

Almost as soon as war was declared, Mira’s lover, Willes, alerted the authorities about their conversations. He had heard that her husband was in the Royal Naval Reserve on the Kent coast and was concerned that he might pose a threat to national security.

Disagreements inevitably arose between MI5 and Naval Intelligence – the former suggesting to the Admiralty that they had a potential enemy in their ranks who should be immediately relieved of sensitive duties. The Admiralty ignored them.

The war continued, but little progress was made in discovering more about the suspected spies.

While MI5 pointed out that one of Dmitri’s best friends was a known drug addict – making the Prince a security risk – the Navy promoted him to Lieutenant Commander.

The spies retaliated by plotting Dmitri’s London address, noting that Mira spent a lot of time at The Ritz, the Embassy Club, and the Berkeley, complaining about free meals from her wealthy friends.

MI5 didn’t have the guts to bring the pair in for questioning, considering they mingled with important and powerful people. So out of desperation, spy chief Knight – who wanted to have the princess arrested – visited Wolkoff in Aylesbury prison.

“All I can tell you is that she is very beautiful, very stupid and drinks like a fish,” was the traitor’s first answer to his questions. She later opened up about the infamous Austrian trip in 1936.

“She’s a fun-loving woman who would do anything for her creature’s comfort, and perhaps for money,” Wolkoff said of the princess. “She’s amoral and unloyal.”

By early 1942, the couple’s surveillance had run out. There was no evidence of espionage.

An MI5 case officer admitted they were “satisfied” that whatever activities the princess had been involved in in the past, neither she nor her husband was up to anything sinister.

Mira and Dmitri’s marriage later failed and the princess applied for a visa to go to the United States.

With her application approved by two cabinet ministers, MI5 was powerless to stop her. With her daughter, she started a new life in New York, took a job as a sales assistant and remarried twice.

She died at the age of 56 in January 1969 in Sharon, Connecticut.

Yet she had been under surveillance for years – while the British Embassy in Washington sent MI5 a top-secret report on Mira’s whereabouts and behavior.

They should have saved themselves the trouble. The Frogmore Cottage traitors were as effective at spying as a mournful pair of blue-blooded money-grubbers.

• Hitler’s Aristocrats: The Secret Powers in Britain and America Who Supported the Nazis, by Susan Ronald, Amberley Publishing, £25.