More than 400 former jihadi fighters back in Britain, say security sources

UK authorities are facing an increased terror threat from battle-hardened fighters returning from Mosul and other conflict zones in Iraq and Syria.

Security sources have told Sky News more than 400 former fighters are now believed to be back in Britain.

The authorities believe there is a growing risk the UK could suffer the kind of mass gun and bomb attacks seen in France and Belgium recently, as many returning fighters will have been trained in the use of weapons and the construction of improvised explosive devices.

It is a serious, two-pronged challenge for the police and security services, who are already working flat-out to counter the threat from homegrown lone-wolf extremists, like Khalid Masood, who launched last week's deadly attack on Westminster.

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Former Scotland Yard Specialist Firearms Officer and author Tony Long said combating an attack launched by a well-trained returning jihadist could be a tough prospect.

He said: "These are combat-hardened soldiers. They might not be trained in the way that NATO might train their soldiers but they've seen more close quarter conflict and more urban fighting than probably most members of the British Armed Forces and you have to respect that.

"Of course they're bringing that knowledge back with them to the UK and it's very very difficult because of the legal restrictions that are put on the security services and the police to actually monitor all of these people."

To date, only a fraction of those returning from the battlefields of Syria and Iraq have been prosecuted, as authorities need enough evidence to put before the courts and often returning fighters go to great lengths to cover up their overseas activities.

Imran Khawaja, 29, from west London, is currently serving 12 years in prison after he faked his own death in Syria in an attempt to sneak back into the UK undetected.

Khawaja had joined a militant group with links to so-called Islamic State while overseas.

He was pictured posing with the severed heads of Syrian soldiers during his six months in the country.

He was arrested as he tried to re-enter the UK through the port of Dover and later admitted preparing for acts of terrorism, attending a camp, receiving training and possessing firearms.