Renewed doubt has been cast on a claim by Adolf Hitler’s favourite architect that he once considered assassinating the Fuhrer by the discovery of a long forgotten Foreign Office document.

Albert Speer, who also served as Nazi minister for armaments and war production, claimed he turned against Hitler in the latter stages of the Second World War and planned to assassinate him by pumping poison gas into the Fuhrer Bunker in Berlin.

Historians have long doubted the claim, which helped Speer escape a death sentence in the Nuremberg trials at the end of the Second World War.

But now a German historian claims he has found new evidence it is false in a memo preserved by the Foreign Office. The memo, from an American official who interrogated Speer before the Nuremberg trials, appears to show key details of the architect’s story changed over time.

Speer was one of the most senior Nazis to stand trial after the Second World War. He claimed he didn't know about the Holocaust or other Nazi crimes and expressed remorse over his role in the regime, and was sentenced to 20 years in prison.

Prof Jürgen Brautmeier of Düsseldorf University says he found the memo in the National Archives in Kew. It appears to have been preserved by a junior Foreign Office diplomat, named only as Lawrence, who was also present at the interrogation.