For almost 60 years the story of the secret listeners who bugged the Nazis remained a closely guarded secret hidden within heaps of unseen government files.

These documents, compiled by British intelligence services throughout the Second World War, only became publicly available after their official declassification in 1999.

But it wasn’t until Golders Green historian Dr Helen Fry began trawling through the transcripts in 2009 that the work of the German refugees who listened in on Nazi prisoners of war came to light.

Earlier this month, Dr Fry released The M Room: Secret Listeners who Bugged the Nazis, a book dedicated to the secret listeners’ story and revealing for the very first time how British intelligence services knew about the Holocaust during the Second World War and chose not to provide evidence to Nazi war crime trials.

For three years, Dr Fry picked through almost 100,000 documents held at The National Archives, in Kew, to discover more about the secret listeners after being contacted by Fritz Lustig, 93, one of only two surviving secret listeners and a contributor to the book.

“If it wasn’t for Fritz’s testimony this story may have remained undiscovered,” said Dr Fry.

“What we don’t get from the documents at The National Archives is the secret listeners’ story. We don’t get the human dimension.

“These were not ordinary listeners, these were German Jews that had fled Nazi Germany.”

Mr Lustig, who now lives in Church Crescent, Muswell Hill, arrived in England from his native Germany in April 1939, as a Jewish refugee fleeing Hitler’s murderous regime.

Initially, the amateur cellist found himself a member of the British Army’s Pioneer Corps orchestra but by 1943 he had transferred to the Intelligence Corps, where he joined fellow refugees listening to bugged conversations between German prisoners of war.

This bugging operation took place across three stately homes, in Enfield and Buckinghamshire, and recorded the conversations of more than 10,000 prisoners of war held at the homes.

But it was not until the operation was brought to Dr Fry’s attention, that the transcripts of these conversations and their sensational content were finally revealed.

“They picked up really graphic stuff about war crimes and in particular the extermination of the German people,” said Dr Fry.

“It tells us for the very first time that the German army and the generals took part in the extermination of European Jews.”

Among the most gruelling conversations revealed in the book was a discussion between German army generals Dietrich von Choltitz and Wilhelm Ritter von Thoma in which secret listeners recorded Choltitz remarking: “The worst job I ever carried out - which however I carried out with great consistency - was the liquidation of the Jews.”

The London Jewish Cultural Centre (LJCC), in North End Road, Golders Green, will be hosting the official launch of Dr Fry’s book on January 29, where she will be joined by Mr Lustig.