The diary of Nazi doctor Josef Mengele, who experimented on victims at Auschwitz, has been bought by the grandson of a Holocaust survivor.

Known as the “Angel of Death” by the camp’s inmates, Mengele conducted gruesome experiments on inmates, particularly on twins, and ordered the deaths of prisoners with medical problems.

The diary, written when Mengele was in hiding in South America after the war, contains his rants on “racial purity” and the “staining of bloodlines”.

He wrote of the need to define what kind of human life is worthwhile and the need to sterilise those with “deficient genes”.

“Everything will end in a catastrophe if natural selection is altered to the point that the gifted people are overwhelmed by billions of morons,” he wrote.

Bill Panagopulos, an auctioneer of historic documents from Alexander Autographs in Connecticut, said he had sold the 180-page journal to an “East Coast Jewish philanthropist”.

Mr Panagopulos told the Hartford Courant: "He is the grandson of an Auschwitz survivor who personally encountered Mengele at Auschwitz. He intends to donate the manuscript to a museum devoted to the Holocaust.

“I am overjoyed that the manuscript is going where it belongs, where it will be available to historians and scholars.”

He added: “Make no mistake about it — I have no sympathy for these monsters. My father's home town was wiped out by the Nazis in a reprisal action.

"It is of vital importance that such documents remain available as tangible evidence of the evil deeds of the past."

Mr Panagopulos said he was certain the journal was authentic and matched the handwriting of the Nazi doctor.