His role in one of the most notorious scandals of the 20th century has long been a source of speculation.

Now the Mail on Sunday can reveal that Prince Philip was named in top-secret FBI documents about the Profumo affair in the early 1960s.

The papers show the FBI had been told the Duke of Edinburgh was personally 'involved' with Christine Keeler and Mandy Rice-Davies, the two women at the centre of the sex scandal that brought down the government.

Files from the US Department of Justice show the crucial memo was written by J Edgar Hoover, then director of the FBI – and it can be revealed after a five-year quest by the MoS seeking relevant documents under American freedom of information laws.

At the heart of the scandal was the married secretary of state for war John Profumo, who had denied in the Commons in March 1963 that he had a sexual relationship with teenage showgirl Keeler. He was forced to resign months later when proof of the affair became public.

The scandal had national security implications because Keeler was also sleeping with Russian military attache Yevgeny Ivanov. 

In the fallout, well-connected osteopath Stephen Ward, who had introduced Keeler to Profumo at a party, was charged with living off the immoral earnings of Keeler and her friend Rice-Davies. He took a fatal overdose and died three days after he was convicted.

In the newly disclosed document, Thomas Corbally, a US businessman involved in industrial espionage, who agreed to be interviewed by the FBI about his friendship with Ward, makes the claim about Philip. 

A cable sent by Hoover to the US embassy in London on June 20, 1963 reads: 'Corbally also stated there was a rumour Prince Philip may have been involved with these two girls.'

Philip's links to the Profumo affair formed a key storyline in the Netflix series The Crown.

Sir Anthony Blunt, the Queen's curator of art later disclosed to be a Soviety spy, warns Philip that he will expose his relationship with Ward when the Duke tackles him over his treachery.

Philip had certainly had been in contact with Ward on a number of occasions and had even been sketched by the artist at Buckingham Palace 

In his evidence to the FBI, Corbally said he did not believe the charges against Ward.

Ward was was an unlikely associate of Christine Keeler or her friend Mandy Rice Davies. 

Keeler found herself living with Ward - in a non-sexual relationship - both at his London apartment in London and at his weekend cottage on the grounds of the Cliveden estate, home to the 3rd Viscount Astor. 

It was at one of many Cliveden pool parties that Ward fatefully introduced Keeler to John Profumo, the Secretary of State for War, and his wife, the actress Valerie Hobson 

There matters might have rested but for the fact that Keeler also claimed to have slept with a man called Yevgeny Ivanov, the Soviet naval attaché - a potential security risk.

With rumours of his own affair becoming steadily more widespread, Profumo felt obliged to deny it in the House of Commons in March 1963, a lie which sealed his fate. Famously caught out, he resigned the following June.

Thanks to his association with Keeler, meanwhile, Ward faced police charges of immorality. Keeler was neither a prostitute nor an ‘escort’, but certainly did live off the generosity of wealthy men - and attitudes, then, were censorious.

Two days later, Ward was arrested and formally charged with living off immoral earnings - a charge for which, however louche he might have been, there was little evidence.

Keeler had served four and a half months for perjury after being found to have falsely accused a man of assault at a friend's home. 

Unseen risque pictures of Keeler emerged in 2019, two years after the death of the 1960s glamour icon. 

The images of Keeler are believed to have been taken soon after the scandal, one in 1966 of her wearing only a pair of trousers.

Another even more revealing picture, seemingly from the same shoot, depicts a 24-year-old Keeler in a matching outfit with her bare back to the camera.

The images were taken by an unknown photographer while Keeler worked as a showgirl at the glamorous Murray's Cabaret Club in Soho, London.

A third photo, taken earlier in 1964, shows Keeler eating a chicken drumstick following her release from prison. 

Who was Christine Keeler and what was the Profumo affair?

Born in Middlesex, Christine Keeler moved to London as a teenager and began working at Murray's Cabaret Club in Soho. 

It was there she met Dr Stephen Ward, a high-flying London osteopath and fixer who ‘procured women’ for leading members of the Establishment, who introduced her to Conservative Minister John Profumo while at a party thrown by Lord and Lady Astor in 1961.

The pair hit the headlines after seven shots were fired at Ward's house in a quiet Marylebone mews by a jilted boyfriend of Keeler a year later in December 1962.

It emerged the then 19-year-old Keeler had been sleeping with former Secretary of State for War John Profumo, then 48, and at the same time a handsome Russian spy Evgeny Ivanov.

But when the news broke, Profumo lied to the House of Commons about his affair. He was soon found out and Keeler sold her story to the News of The World for £23,000.

In June 1963, he quit in disgrace, amid allegations Keeler had been asked by Ivanov to discover from the War Minister when the West Germans might receive U.S. nuclear missiles to be stationed on their soil.

Profumo had been a rising star of the Tory Party, close to Prime Minister Harold Macmillan, a favoured visitor at Buckingham Palace, a war hero and the dashing husband of actress Valerie Hobson, one of the great beauties of her day.

Ms Keeler, whose other lovers have included A-Team actor George Peppard, legendary womaniser Warren Beatty and Prisoner of Zenda star Douglas Fairbanks Jr, said in an interview years later that the Establishment was far more interested in painting it as a sex scandal and chose to ignore whispered claims of a widespread spying network. 

Christine died aged 75 in December 2017.