Usk-based author John Humphries has released a new book based on the life of Welsh spy Gwilym Williams.
The non-fiction book, "Spying for Hitler: The Welsh Doublecross", is being published by the University of Wales Press at £19,99, and was released on September 1 (Sat).
It chronicles the life of the Swansea-born police inspector who was recruited by MI5 and infiltrated German military intelligence, helping to ttip the balance of the esponiage war in BritainÕs favour.
Mr Williams posed as a Welsh Nationalist collaborator and successfully tricked the Nazis into believing he was working for the downfall of Britain, when in fact he was reporting to MI5.
Mr Humphries said: "An associate of mine was writing about IRA collaboration with the Germans, and told me about this little-known Welsh spy.
"Even after much research it took me a long time to establish who he was. He had been redacted from almost all the records, and instead was simply referred to as "GW". But eventually I found they had forgotten to cross his name out on just one page, and his name was Gwilym Williams.
"I discovered how he had been interrogated by German intelligence, who were satisfied he was working for them, when in fact he was working for MI5 the whole time, infiltrating the Germans.
He discovered an espionage ring operating from the Spanish embassy in London, and cracked it."
Mr Humphries, a former editor of the Western Mail, went through hundreds of declassified files and documents in his search to find out more about Mr Williams.
Eventually, he discovered Mr Williams in considered one of the founding members of the Double-Cross System, a World War II anti-espionage and deception.
During World War II MI5 would capture German agents and either shoot them or turn them into double agents to feed misleading information to the nazis.
Mr Humphries said: "He and another Welsh man were the founding members of this system. They are credited with winning the esponiage war in Britain."