Many Britons who have returned from Islamic State territory are not being prosecuted and instead need to be reintegrated because they were simply naive, the Government’s terrorism laws watchdog has said.

Hundreds of Britons who went to Syria or Iraq in the past few years are already thought to have returned, but the authorities have decided not to charge all of them, Max Hill QC said.

He spoke days after Andrew Parker, the director general of MI5, said the country faced an intense threat from Islamist terrorism, with plots now being hatched at a tempo he had never seen before.

Around 850 Britons are thought to have travelled to Syria since 2011. Around 120 are believed dead fighting for groups including Islamic State and around half those remaining are thought to have already returned home.

Police and security leaders have in the past warned of the prospect of battle-hardened jihadists returning to continue their struggle by carrying out attacks on the streets of Britain.