Britain is planning to send 400 more troops to Afghanistan to help tackle the growing threat posed by extremists - a number that could rise and be used for combat, a former army commander has said.

Gavin Williamson, the defence secretary, is understood to be keen to nearly double the number of soldiers in the country as part of a Nato training mission.

It comes nearly two decades after Britain first deployed ground troops to Afghanistan in the aftermath of the 9/11 terror attack, and nearly four years after combat soldiers were withdrawn as Camp Bastion was handed over to Afghan security forces.

General Sir Richard Barrons, who served as commander of Joint Forces Command, said the decision to withdraw from Afghanistan in 2014 "hasn't worked" and supported sending 400 extra troops back to Kabul.