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.
ISIS views Britain's European Union referendum in June with "great interest" and is likely planning an attack in the UK which would persuade voters to leave the EU, he said.
"If the referendum result leads to the UK leaving the EU, ISIS will take credit for having struck the first significant blow against the EU which it sees as representing the nations that carried out crusades against Muslim states for control of the Holy Lands between 1089 and 1390," he said.
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Dean's transformation came after the 1998 al-Qaeda attacks on the US embassies in Kenya and Tanzania, which left 224 dead -- all but 12 were local civilians.
He disagreed with al-Qaeda's justification for killing civilians and went from al-Qaeda activist to "betrayer", with the help of the secret services of Qatar and then the UK.
Dean, who advises both public and private sectors on terrorism from his base in Dubai, also claims that a civil war within Muslim communities is driving the west towards further Middle East conflict.
"The West is the side casualty," Dean said.