A Holocaust historian this week came to the defence of a British prisoner of war whose account of his time at Auschwitz has been called into question.

Denis Avey, 93, recorded his experiences in The Man Who Broke into Auschwitz, in which he recounted how he had swapped places with a Jewish prisoner so he could see the death camp for himself.

An article by Nicholas Hellen in this week's Sunday Times claimed that important details in the book differed from those he had given 10 years ago in an interview lodged at the Imperial War Museum. It alleges that the name of the camp had changed, as had the name of the prisoner with whom Mr Avey swapped places.

But Lyn Smith, who conducted the original interview for the museum in 2001 and has included Mr Avey in her forthcoming book Heroes of the Holocaust, to be published in January, insists that he is an utterly reliable witness.

She said: "I was sent by the museum to ask about his experiences in Auschwitz. He had never spoken about any of these things before and I caught him on the hop. I don't suppose he thought for a moment he was going to talk about the things he did with me".

She added: "People do sometimes makes mistakes with names, particularly when it's spontaneous and in the middle of an interview – this is to be expected. It was not my job to double-check the places and names he gave to me. But the author of the book, Rob Broomby, would have double-checked everything."

Ms Smith said Mr Avey was "a very upright person. I wouldn't doubt him for a moment. He's a very credible witness. It's pitiful what happened to him."

She worried that the Sunday Times article might have unintended consequences. "Holocaust deniers may pounce on this and say that people have made up the whole thing. That's what distresses me. But these were genuine mistakes about events which happened a long time ago."

Mr Avey said: "I understand that some people have found my story hard to believe and I have always done my best to answer every query put to me as fully as I am able. I am sad that, even so, a few people are determined to undermine and discredit my account".

His publishers, Hodder & Stoughton, said: "We had no reason to doubt Mr Avey's account at the time of publication - and we have no reason to doubt it now".