Europe faces a generation-long struggle to deal with thousands of battle-hardened jihadists returning as the Islamic State’s caliphate starts to collapse, the EU’s police chief has warned.

An estimated 3,000 European fighters are believed to be with Islamic State in Iraq and Levant (Isil) with officials fearing the coming battle for Mosul will see many try to return home.

Around 850 Britons are thought to have gone to fight in Syria or Iraq, with around 400 still in the country.