A former al-Qaeda fighter who later became a spy for the British secret service says ISIS views the Britain’s European Union referendum in June with “great interest” , and is likely planning an attack in the U.K. which would persuade voters to leave the E.U.

Aimen Dean—who once swore allegiance to Osama bin Laden in person—tells TIME in an interview in London that Islamist extremists in al-Qaeda and ISIS would see the U.K.’s departure from the E.U. as a first step in the destruction of the union, which they see as a successor to the Roman Empire.

Much of the ideology of ISIS and al-Qaeda is rooted in theories of a clash of civilisation between Muslims and the non-Muslim world, which were formulated in the 7th century when Muslims fought for supremacy with the Byzantine (Roman) Empire and the Persian Empire.

Dean says that recent attacks in Brussels and Paris are also part of a strategy to destroy non-Islamic institutions and states and provoke conflict between Muslim and non-Muslims in the Middle East. This final battle, they believe, has been foreseen in religious writing and will herald the end of the world.