There is a “very high risk of a terrorist attack” against the evacuation operation by the UK, US and other allied forces in Kabul, according to a senior British source.

The group that is of most concern is an Islamic State affiliate in Afghanistan called Islamic State Khorasan, or ISIS-K, the source said.

The kind of attacks they are known for conducting in particular are suicide bombings such as a car bomb or an individual blowing themselves up.

UK, US and other allied forces are particularly exposed because they do not have complete control over their own security, instead having to rely on the Taliban to help secure access to the airport.

In recent days, ministers, including the defence secretary and armed forces minister, have raised concern about the security threat to troops involved in the evacuation effort.