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Police are investigating a sensational claim that the SAS was involved in the death of Princess Diana, reports Sean Rayment.

The allegation emerged at the second court martial of Sergeant Danny Nightingale, who was found guilty of illegally possessing a gun and ammunition.

It came in a letter to the elite unit’s commanding officer by the parents-in-law of a special forces sniper, known only as Soldier N, who was Sgt Nightingale’s former housemate and a key witness for the prosecution.

The letter said Solider N boasted the SAS “was behind Princess Diana’s death”.

The Ministry of Defence, the Royal Military Police and the Service Prosecuting Authority have known of its existence and the claim since it was sent in September 2011.

The seven-page hand-written letter – a copy of which has been seen by the Sunday People – also makes allegations over Soldier N’s behaviour towards his wife and her family following the collapse of the couple’s marriage.

It is understood that the SAS passed the letter to the Service Prosecuting Authority prior to the start of the Sgt Nightingale trial.

All references to the paramilitary force were removed by the SPA before it released the document to the court.

The letter says: “He (Soldier N) also told her (his wife) that it was the XXX who arranged Princess Diana’s death and that has been covered up.”

Scotland Yard said: “The Metropolitan Police Service is scoping information that has recently been received in relation to the deaths of Diana, Princess of Wales and Dodi Fayed and assessing its relevance and credibility.

“The assessment will be carried out by officers from the Specialist Crime and Operations command.”

The former Harrods owner Mohamed Fayed has long maintained the car crash that killed Diana, his son Dodi – who was her lover – and their driver Henri Paul in Paris on August 31, 1997, was a conspiracy involving the British state.

A 2008 inquest jury returned an unlawful killing verdict.

It was a tip-off from Soldier N’s estranged wife that led to a Glock 17 pistol being found in Nightingale’s room at a Hereford house the SAS colleagues shared.

Soldier N pleaded guilty to possessing another Glock, a grenade and ammunition. He was sentenced to two years in jail.

Nightingale, 38, from Crewe, Cheshire, was sentenced to two years’ military detention, suspended for 12 months, after being found guilty at a retrial last month.

His initial 18-month sentence was quashed after a campaign by wife Sally.

The MoD said: “This is a matter for the civilian police authorities.”