“Syria, Libya, Mali and Somalia are being ravaged by jihadist outfits, and all of them are attracting European-born Islamists seeking the thrill of real combat…

“Scores of young European-born Arabs and Somalis are following in the footsteps of British Pakistanis in travelling to lawless conflict zones.

“What happens when these men, schooled in the use of political violence in far-flung places, return to Britain?”

Five people, including two teenagers, were arrested in London this month after a group calling itself the Muslim Patrol posted footage on YouTube showing a gang hurling abuse at a homosexual man and described white women as “naked animals with no self–respect”.

But their alleged offences could be just the tip of the iceberg, according to Mr Nawaz.

“The Muslim patrols could become a lot more dangerous and, perhaps willing to maim or kill if they are joined by battle-hardened jihadis,” he wrote.

He compared the Islamist vigilantes to extremists like the far-right Golden Dawn supporters in Greece and right-wing vigilantes in France who ran Roma families out of a Marseilles estate and burnt down their camp.

Countries such as Denmark and Spain have also seen Islamist extremists trying to enforce their own sharia law, he noted.

All were imitating Hitler’s Brownshirts by “enforcing with threats and violence their version of the law in neighbourhoods,” said Mr Nawaz, who spent years in his youth as a leadership member of a global Islamist group.

Mr Nawaz is now a leading critic of his former Islamist ideological dogma, while remaining a Muslim.