A gangster was detained during World War II as it was believed he could be used by Italy to lead an insurgency in the UK, according to secret documents.
Ottavius Sabini terrorised racing tracks with blackmail, extortion and protection rackets in the 1920s, files from the National Archive reveal.
He was arrested in 1940 by Scotland Yard who thought Italian secret agents would use him to lead an uprising.
But the Home Office said he had no political links and was released.
Police went to Hove in Sussex and arrested Mr Sabini - who had the aliases Darby Sabini and Frederick Handley - at the age of 53 under Defence Regulation 18B, the documents released on Monday reveal.
The Scotland Yard report describes Mr Sabini as "a drunkard and a man of most violent temperament with a heavy following and strong command of bullies of Italian origin and other undesirables".
It adds: "He can be described as a dangerous gangster and a racketeer of the worst type and one whom it is most likely enemy agents would choose as a person to create and lead violent internal action against this country."
But the Home Office was not convinced and in its notes points out Mr Sabini was the son of an Italian father and British mother, had a British wife and a son serving in the RAF.
Its report says although Mr Sabini led a "nefarious existence" in the underworld of boxing and gambling, there was no evidence he belonged to any political organisation or had any connections with Italy.
It adds: "In fact for the purposes of Regulation 18B there does not seem ever to have been a very strong case against Sabini other than that he was a low type person of Italian origin and an unscrupulous 'tough' who might do anything for money."