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When Welsh bobby Gwilym Williams retired after 29 years, it was thought his finest deed was stopping a runaway horse.

But it has now been revealed he went on to lead an amazing secret double life – by spying on the Nazis during the Second World War.

The inspector posed as a fanatical Welsh nationalist and tricked German spy chiefs into revealing secrets, which he then passed on to MI5.

Gwilym uncovered a plan to land a German U-boat on a South Wales beach, a scheme to steal a Spitfire and a plot to pour poison into the Cray Reservoir near Brecon.

His escapades are detailed in a new book called Spying For Hitler by John Humphries, who said: “John Masterman, chairman of the Twenty Committee which ran the double-cross system, regarded Gwilym Williams as Britain’s best agent.”

Gwilym died aged 62 in 1949, but the author learned of his story in declassified security files at the National Archive.

They revealed he left the police force in his home city of Swansea with an undistinguished record.

He had been reprimanded for being drunk on duty and assaulting residents.

But in September 1939, MI5 sent him to Belgium to infiltrate the Abwehr, Hitler’s spy service.

The book tells how Gwilym was recruited after spy chiefs learned of German plans to forge links with Welsh nationalists.

They decided to exploit the exaggerated view the Nazis had of Welsh anti-English feelings and groomed Gwilym as a double agent.

His only training was to memorise the names of prominent members of the Welsh nationalist party.

He was sent to meet his German handlers in Antwerp, where he was asked to used his “nationalist network” to sabotage aerodromes, power stations and munitions factories.

He was offered £50,000 to arrange for one of Britain’s “latest aircraft” – the Spitfire – to be flown to France so the Nazis could uncover its weaknesses.

Like all double agents, Gwilym risked being tortured if caught.

Mr Humphries added: “He had the Abwehr jumping through hoops and helped us win the intelligence war.”