A little-known German foreign ministry official described by the CIA as the most important spy of the Second World War will be honoured by his government 30 years after he died as an outcast.

Fritz Kolbe was hailed by America after passing more than 1,600 top-secret Nazi documents to US agents based in Switzerland. But in Germany he was seen as a traitor for years after the war and his name is still absent from history books.

Now a new book has begun the process of rehabilitating Kolbe and is set to earn him a posthumous award from the German foreign ministry.

Fritz Kolbe, the Second World War’s Most Important Spy, based on CIA documents declassified four years ago and Kolbe’s private letters, details for the first time his attempts to ensure that Hitler lost the war.

“My aim was to help shorten the war for my unfortunate countrymen and to help concentration camp inmates avoid further suffering,” he wrote in a letter from his home in Switzerland in 1965. He never received any money for his spying.

The book, by Luca Delattre, a French journalist, describes how Kolbe, codenamed George Wood, gave secrets to the Office of Strategic Services, the CIA’s predecessor.

Richard Helms, a former head of the CIA, said Kolbe’s information was “the most important ever supplied by an agent working for the Allies” during the war. Kolbe’s handler, Allen Dulles, who also once headed the CIA, acted as a referee for him after the war when he desperately tried, but failed, to clear his name. He eventually went to Switzerland where he became a power-saw salesman in Bern.

“The risks Kolbe took were uncalculable,” Dulles wrote. “I just hope that the injustice done to him will be reversed one day and that his country recognises his true role.”

The foreign ministry in Berlin, which helped M Delattre to research his book, said it was considering how to honour Kolbe and several of his colleagues in the anti-Nazi resistance.

Kolbe first approached the British who allegedly laughed at him but by 1943 only 11 people, including President Franklin D Roosevelt, were allowed to see his documents.

Among files Kolbe passed on were reports on what Berlin knew of Allied movements and plans for submarine attacks on Allied convoys.

Kolbe was ostracised after the war because the foreign ministry under Konrad Adenauer was full of former Nazi diplomats who despised him.