Writing in 1971 to Hélène Jeanty, the widow of a Belgian resistance leader, Speer admitted that he had been at a conference where Heinrich Himmler, the head of the SS and Gestapo, had unveiled plans to exterminate the Jews in what is known as the Posen speech. Speer's insistence that he had left before the end of the meeting, and had therefore known nothing about the Holocaust, probably spared him from execution after the Nuremberg trials at the end of the second world war.

It helped earn him the name of "the good Nazi" and the image of a genius architect who had misguidedly slipped into Nazi circles to further his career. Instead of facing death as many top Nazis did, Speer served 20 years in prison, mainly for using slave labour.

In the letter to Jeanty, written on December 23 1971, Speer wrote: "There is no doubt - I was present as Himmler announced on October 6 1943 that all Jews would be killed". He continued: "Who would believe me that I suppressed this, that it would have been easier to have written all of this in my memoirs?"

Speer, who died in London in 1981, denied knowing about the Holocaust in his best-selling 1969 book, Inside the Third Reich, as well as in lengthy interviews with the British author Gitta Sereny, who wrote a biography on him.

The letter is part of a collection of 100 between Speer and Mrs Jeanty, an author, written between 1971 and 1981, recently found in Britain. They are due to be auctioned at Bonhams, London, on March 27.