An auction house is to investigate whether a letter it is selling, sent by a Nazi official about the Holocaust, is a fake.

Shropshire auctioneers Mullock’s, plans to sell the letter on August 21. It will be auctioned alongside Hitler’s personal copy of the German version of the “yellow pages” and a wedding-gift edition of Mein Kampf.

Postcards showing photographs of dead Jews in Auschwitz, taken by a Russian soldier, are among dozens of Nazi-related items expected to fetch a total of over £80,000.

The letter, dated 19 July 1942, was sent by German railway boss Dr Theodor Ganzenmüller to Heinrich Himmler’s chief of staff Karl Wolff, detailing the transportation of 5,000 Jews daily to the Treblinka extermination camp.

It was written in response to complaints by Himmler, the Final Solution’s architect, that the German railway authority was not transporting Jews to their deaths fast enough.

But the document, expected to fetch £600, is an almost identical copy of another well-documented letter used in evidence at the Nuremberg Trials, which is dated July 28 1942. There are significant inconsistencies between the two letters.

Ben Barkow, director of the Wiener Library, one of the world’s most extensive archives on the Holocaust, said the Mullock’s letter is troubling because its wording and grammar “are riddled with mistakes”.

He said: “Without a satisfactory account of the history of this document since 1945, I believe that few historians would feel able to have confidence in its authenticity”.

Mullock’s expert Richard Westwood-Brookes said: “I shall be investigating this with the vendor and making enquiries. We are obviously putting it up for auction in good faith. If I can’t get to the bottom of this we will withdraw it pending further investigations.”