Survivors of the Mauthausen concentration camp cheer the soldiers of the Eleventh Armored Division of the U.S. Third Army one day after their actual liberation in May 1945. Photo: National Archives and Records Administration.

A wartime refugee has revealed in a new book how he was recruited by British intelligence to eavesdrop on Nazi prisoners.

Fritz Lustig was one of about 100 refugees, mostly German Jews, hired to listen in on conversations between officers in bugged POW cells around London.

“We were working in two shifts from 8am to 4pm and then from 4pm until prisoners went to bed,” he told the London Evening Standard.

“As soon as we noticed that something important was being discussed we cut a record. The records we kept of atrocities committed were given a special marking in red. We knew these things were happening. I was fortunate because all my closest family had got out of Germany. The prisoners never saw us and we never saw them, I didn’t even know where their cells were.”

The operation is gone into in further detail in a new book by historian Helen Fry, “The M Room: Secret Listeners who Bugged the Nazis.” It details how Lustig, and others like him, gleaned information on the Holocaust from over 10,000 prisoners. Fry, who sifted through over 100,000 documents, told the London Evening Standard that one of the most chilling was that of General Dietrich von Choltitz, the former military governor of Paris, saying: “The worst job I ever carried out — which however I carried out with great consistency — was the liquidation of the Jews.”

Fry said: “They heard von Choltitz saying how Jews were lined up in pits in the woods and shot, and children thrown in.”

Lustig, who had never spoken on record about his role in WW2, said: “I had to sign the Official Secrets Act which meant I couldn’t talk about it. It is important that people know what we did.”