IT began with a letter uncovered nine years ago addressed by a duke to “Dear Mr Hitler”. One of the most extraordinary academic detective stories of modern times ended yesterday when the National Archives, the official custodian of Britain’s history, admitted that it had been the victim of a master forger.

The public records office, which holds the written record of the British state going back 800 years, said its reputation had been compromised by the discovery that 29 documents from 12 separate files were all forgeries inserted into its records.

The forged documents all relate to alleged British perfidy in the second world war. The archive says the papers had supported sensational allegations by Martin Allen, a self-styled “eminent” historian, in three recent books. These include claims that the Duke of Windsor was a traitor and that British agents had murdered Hitler’s SS boss, Heinrich Himmler, on Winston Churchill’s orders.

The archive also released witness statements given by senior officials to the police. In one of them, Louise Atherton, an archivist, is particularly critical of Allen, saying he “relied heavily” on forged files and accusing him of “significant” exaggeration and inaccurate quotation in his use of the contents of genuine ones.

Allen was unavailable yesterday. One of his former literary agents, Robert Smith, said that Allen had previously denied any involvement in the forgeries.

Suspicions about the authenticity of documents used by Allen were raised by The Sunday Times eight years ago in relation to his book Hidden Agenda: How the Duke of Windsor Betrayed the Allies.

It accused the Queen’s uncle of helping the Germans to conquer France and defeat the British army in the early stages of the second world war.

Allen’s scoop was based largely on what he said was a handwritten letter from the duke to Adolf Hitler dated November 1939. It begins, “Dear Mr Hitler”, and is signed, “EP”, for Edward Prince, an abbreviation the duke occasionally used.

The letter makes veiled references to a tour of the French frontline defences that the duke had just made. It asks Hitler to pay close attention to information the man bringing the letter to him has memorised.

The courier was a German spy, and Allen argues that through him the duke gave Hitler top-secret strategic information that enabled the Germans to attack France at the weakest points in its defences. France fell in six weeks and the British forces were routed.

The letter appears to suggest that the duke, who abdicated in 1936, was willing to resume the throne once Britain had been bullied into a peace settlement.

The Sunday Times commissioned experts to examine the document. Robert Radley, a forensic documents examiner, found “many discrepancies” that made him “highly suspicious”.

Leslie Dicks, another expert, concluded that the letter was “most probably a forgery”.

A paper analyst, Peter Bower, found evidence the letter had been written on a blank page from an old book, a classic forger’s trick. This and other evidence, Bower said, “all combine to suggest that this letter is a forgery and probably done relatively recently”.

When challenged to explain these findings, Allen insisted the letter was genuine. He said it had been given to his late father, also a writer on the Nazi era, by Albert Speer, Hitler’s former munitions minister. He said he had found it in his attic.

The Sunday Times declined to serialise the book but it was published and The Observer newspaper nominated Hidden Agenda for book of the year.

There the matter might have lain as an unresolved mystery but in 2003 Allen published The Hitler/Hess Deception, which argued that the flight of Hitler’s deputy, Rudolf Hess, in 1941 was part of a plot to oust Churchill. Once again Allen was challenging the standard version of events, and using documents to prove his thesis.

His next book, Himmler’s Secret War, published two years later, reinforced the forgery concerns. It cited documents from the National Archives that purported to show that British agents acting on Churchill’s orders murdered Himmler, the head of the SS, in 1945 in order to cover up British peace feelers towards him. This contradicted accepted accounts that Himmler had killed himself.

Alarm bells soon rang. Letter-heads on the correspondence supposedly written in 1945 had been created on a high-resolution laser printer. Signatures were found to be written over pencil tracings. Handwriting of different officials was suspiciously similar. Diplomatic titles and key dates were wrong.

Once more Allen denied culpability. He told The Sunday Times: “I think I have been set up. But I do not even know by whom. I am devastated.”

The National Archives conducted an investigation which confirmed that five documents used in the book had been forged. The inquiry also discovered more forged documents, cited in The Hitler/Hess Deception and in Hidden Agenda. Once 17 documents had been identified as likely forgeries, Scot-land Yard was called in. Allen was interviewed by police. Again he denied forgery.

Last year Mike O’Brien, then solicitor-general, said in a parliamentary answer the Crown Prosecution Service had decided there was enough evidence against Allen “to provide a realistic prospect of conviction”. The CPS had decided, however, it was not in the public interest to proceed. He said Allen’s “health and the surrounding circumstances” had contributed to the decision. There was no further explanation.

That decision has upset some leading historians. Sir Max Hastings, an expert on the second world war, said: “It is hard to imagine actions more damaging to the cause of preserving the nation’s heritage than wilfully forging documents designed to alter our historical record.”

John Fox, a historian specialising in Nazi Germany, said the National Archives’ statement casts doubt on the veracity of documents in other collections.

“How on earth were these documents slipped in? This is something that the National Archives has to answer. Whoever got these documents in must have done it in a very clever, sneaky way, so you can’t entirely blame the security. But maybe there are questions with the security.”

The National Archives said yesterday that new security procedures had been put in place.

Additional reporting: Anna Mikhailova

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