The Prime Minister is expected to seek a recall of Parliament this week to discuss the worsening situation in Afghanistan according to a Number 10 source.

Timings of the return to Westminster will be confirmed following discussions with Commons Speaker Sir Lindsay Hoyle.

Here we look at some of the key questions.

– How did Britain’s involvement in Afghanistan begin and what was the impact on British troops?

On October 7 2001, almost a month after the 9/11 terrorist attacks in the US, then-prime minister Tony Blair confirmed that British forces were involved in US-led military action against al Qaida training camps and the Taliban regime in Afghanistan. Allied air strikes began.

In November, the first UK troops were deployed to Afghanistan when Royal Marines from 40 Commando helped to secure Bagram airfield.

The 20-year war in Afghanistan saw hundreds of UK personnel suffer life-changing injuries, with more than 450 deaths recorded.

– What is the situation 20 years later?

In April, US President Joe Biden announced the remaining 2,500 US troops would leave by September 11 – the 20th anniversary of the 9/11 attacks.

Other Nato allies then confirmed they would follow suit, including the UK – which in May began withdrawing its remaining 750 military trainers.

Defence Secretary Ben Wallace said Britain had been put in a “very difficult position” to continue the mission once the US announced its decision to leave.

– What did Boris Johnson say about Britain’s final military withdrawal from Afghanistan?

Speaking in July, Mr Johnson said gains had been achieved thanks to a US-led coalition, telling MPs: “We can take pride that Britain was part of that effort from the beginning.

“Over the last two decades, 150,000 members of our armed forces have served in Afghanistan – mainly in Helmand province, which was from 2006 onwards a focus of our operation.

“In the unforgiving desert of some of the world’s harshest terrain – and shoulder-to-shoulder with Afghan security forces – our servicemen and women sought to bring development and stability.

“The House would join with me in commending their achievements and pay heartfelt tribute to the 457 British service personnel who laid down their lives in Afghanistan to keep us safe.”

– What is currently unfolding in Afghanistan?

The country is on the brink of complete collapse amid reports that Taliban fighters have entered the outskirts of the capital Kabul.

Arrangements are reportedly being made to fly the British ambassador Sir Laurie Bristow out of the country.

In what could be seen as a sign of the speed of the collapse, countries were hurriedly removing their embassy staff, as helicopters were seen landing at the US embassy to ferry away remaining personnel.

– What are Mr Johnson and the British Government doing now?

A No 10 source said the Prime Minister was expected to seek a recall of MPs this week to discuss the worsening situation.

Britain is sending 600 troops – including Paras from 16 Air Assault Brigade – on a mission to support the final departure of the remaining UK nationals as well as Afghans who worked with the UK in the country.

Defence Secretary Ben Wallace had previously indicated that they could be there until the end of the month, but given the speed of the collapse that would appear unlikely.

– How are British politicians reacting to what is unfolding and what suggestions have been made about what the UK should do?

There is deep anger among many MPs at the way Afghanistan is 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.

Labour leader Sir Keir Starmer said ministers needed to explain what they intended to do to avert a looming humanitarian crisis and prevent Afghanistan again becoming a base for international terrorism.

Mr Ellwood said it was still not too late to turn the situation around, and called for the despatch of the Royal Navy carrier strike group to the region, urging the Prime Minister to convene an emergency conference of “like-minded nations” to see what could be done.

Mr Tugendhat, who served as an Army officer in Afghanistan, said the priority had to be to get as many people out as possible before Kabul collapsed.