Boris Johnson is facing calls for a last-ditch intervention to prevent the complete collapse of Afghanistan as Taliban fighters were reported to have entered the outskirts of Kabul.

The lead elements of the British force sent to evacuate the remaining UK nationals were understood to be in the capital amid fears it could fall within days or even hours.

In a sign of the speed of the collapse, arrangements were reportedly being made to fly the British ambassador Sir Laurie Bristow out of the country.

It had previously been intended that he should remain in a secure location at Kabul airport along with other international diplomats.

But amid a hurried scramble for safety, helicopters were seen landing at the US embassy to ferry away remaining personnel.

In the UK, there was deep anger among many MPs at the way – 20 years after the first international forces entered Afghanistan – the country was being abandoned to its fate.

The chairman of the Commons Foreign Affairs Committee Tom Tugendhat said it was “the biggest single foreign policy disaster” since Suez, while Defence Committee chairman Tobias Ellwood said it was a humiliation for the West.

Despite the decision of the Biden administration to withdraw the remaining US troops which triggered the collapse, Mr Ellwood said it was still not too late to turn the situation around.

He called for the despatch of the Royal Navy carrier strike group to the region and urged the Prime Minister to convene an emergency conference of “like-minded nations” to see what could be done.

“I plead with the Prime Minister to think again. We have an ever-shrinking window of opportunity to recognise where this country is going as a failed state,” he told Times Radio.

“We can turn this around but it requires political will and courage. This is our moment to step forward.

“We could prevent this, otherwise history will judge us very, very harshly in not stepping in when we could do and allowing the state to fail.”