William Joyce, the notorious voice of Nazi propaganda who broadcast from Berlin and nicknamed Lord Haw-Haw, escaped to Germany before the war despite being under surveillance by MI5 and the Special Branch.
The blunder is revealed for the first time in secret intelligence files which also detail claims that a MI5 officer tipped off Joyce and his wife about their impending arrest.
Joyce, a virulent anti-semite, had been a leading blackshirt in Oswald Mosley's British Union of Fascists. Intelligence officers recommended he should be detained in the event of hostilities.
A month before he fled, the security services intercepted a letter from Joyce to a suspected German spy, journalist Christian Bauer, disclosing his intention to leave Britain for Germany.
Joyce's sister Joan has revealed the MI5 warning. The MI5 officer admitted speaking to him, but denied warning him.
Joyce broke with Mosley, after accusing him of being too moderate, and formed his own National Socialist League.
The security services stated: "He has identified himself unreservedly with the Nazi cause, maintains close contact with Nazi officials and has shown he would be quite willing to take action inimical to this country."
On 1 September, with war inevitable, a detention order for Joyce was issued and Inspector Keeble of Scotland Yard's Special was sent to arrest him. But Joyce and his wife had caught the boat train for Ostend. In May 1945, he was captured on the Danish border after a British officer shot him in the buttocks. The following January he was executed for high treason.
Public Record Office documents also reveal that German attempts to incite IRA attacks in wartime Britain may have been scuppered by a feud between two IRA leaders.
In 1940, a German U-boat was to have landed the fugitive IRA chief-of-staff Sean Russell and his colleague Frank Ryan in Ireland. But Russell was taken ill and died. The cause of his death remained a mystery.
But Lt Col Arhim Lahousen, the head of the German sabotage section, the Abwehr Department II, told Allied interrogators after the war that he believed Ryan poisoned Russell. Intelligence experts believe Lahousen may have been trying to deflect suspicion from his own organisation whose head - Germany's intelligence chief Admiral Canaris - was thought by some to have been responsible for Russell's death. Canaris - executed in 1945 as the war ended for the 1944 bomb plot to assassinate Hitler - is also believed to have worked for the Allies.
The Irish operation was the brainchild of Foreign Minister Joachim von Ribbentrop who was responsible for having Russell brought to Germany after he was expelled from the US.
While Russell was being schooled in German sabotage techniques, the Abwehr managed to get Ryan released from a Spanish jail. He had been captured fighting for the Republicans in the civil war.
The Germans hoped the reunited IRA men would help attacking Luftwaffe bombers by radioing back weather reports.
The communications code was not sophisticated. Russell was to be warned of a German invasion of Britain by "a red potted plant in the window of the German Embassy in Dublin".Reuse content