ISIS is viewing Britain's EU referendum with "great interest" and is likely planning an attack in the UK to persuade people to vote for exiting the powerful bloc, a jihadi-turned British spy has claimed.

Aimen Dean, who once swore allegiance to Osama bin Laden in person, said Islamist extremists in al-Qaeda and ISIS would see the UK's departure from the European Union as a first step in the destruction of the grouping, which they see as a successor to the Roman Empire.

Whatever the Islamic State can do to break up that empire is justifiable to them, said the former jihadist, who became disillusioned with al-Qaeda in 1998 and acted as double agent until 2006, when his name was revealed in a book.