Britain must be prepared to fight a future insurgency unless Isil is wiped out in Syria, the Defence Secretary has warned.

The Islamic State of Iraq and the Levant (Isil) insurgent group is not fully defeated, Gavin Williamson says, and if allowed to escape their “poisonous ideology” will cause further devastation if the momentum to destroy them falters.

Although virtually absent from Iraq and significantly degraded in Syria, Isil fighters remain hidden in unpopulated desert areas including caves.

From around 50,000 fighters in 2014, the insurgents are now thought to number only about 2,000.

But they are still launching brutal attacks on civilians including beheadings, burning people alive and dragging victims behind motorbikes.