A defence expert says the UK Government will want to avoid a "Saigon moment" as troops head to Afghanistan to help British citizens and others escape the deteriorating security situation.
Some 600 British troops are being sent to Afghanistan, tasked with rescuing about 5,000 embassy and aid workers, contractors, and Afghans who worked as interpreters for British forces, along with their families.
Defence analyst Professor Michael Clarke told Forces News: "The Government will want to make it look as normal as possible – the troops are there, the process is under way, the people will go to the international airport, get a civilian flight out to somewhere else and the whole thing proceeds in that manner.
"If the Taliban are sensible they will allow that to happen.
"But if it goes wrong, if it starts to get too difficult to do that, then the thing becomes not just a civilian operation with the military overseeing it, but a military operation where soldiers are rescuing civilians and getting them out in any way they can.
"The danger then is that it starts to look like the fall of Saigon in 1975 and it adds to the narrative of complete failure and complete collapse. The Government will be quite anxious to avoid that impression."
In the final days of the Vietnam War, nearly 7,000 people were evacuated from the US embassy in what was then Saigon, at the end of April 1975.
Watch: What would it take to get US and coalition forces back into Afghanistan?
One of the most famous photos from the time shows a helicopter on the roof of one of the buildings, with a long line of people on a ladder waiting to board – some of them US diplomatic staff but most of them Vietnamese people deemed to be at risk under the incoming communist regime.
Mr Clarke said that, while the plan in Afghanistan was "something the military have got to keep to themselves", it was also likely to have been in existence since British troops arrived there in 2001.
He said it would likely entail telling British nationals and others being rescued where to go either in Kabul, or to get to Kabul, which is one of the few major cities that has not yet been taken by the Taliban.
If anyone cannot get to the capital then it would likely be a case of "let us know where you are, tell us what your circumstances are, and we'll see what we can do", he said.
From there, those being evacuated would probably be taken to the international airport in Kabul and put on a flight from there, likely to Doha, the Qatari capital, or the Al Udeid Air Base nearby.
Mr Clarke said: "The Americans have already put 1,000 extra people into Doha to process their own people, and I can't believe we'd be doing anything different."
However, if the airport in Kabul becomes overwhelmed by Afghans trying to get out, Mr Clarke said troops would need to find another place to stage their evacuation plan.
"The danger is you get to a Saigon moment of 1975 when it looks like a scramble.
"That's the worst way to do it but, in terms of saving lives, that's sometimes what you have to do."
Mr Clarke was confident 600 troops would be enough to protect those being rescued, adding that many of those leaving would be security contractors who "can probably, in a sense, take care of themselves".
Regarding the troops, who are expected to be in theatre within about 48 hours, he said: "It's what they train for, it's what they're ready for.
"Of all the things that Britain does in foreign operations, extraction of nationals is one of the most efficient that we do, because we take it pretty seriously and they train for it really rather well."
The timing of any evacuation would depend on the degree to which Kabul is threatened by the Taliban's advance, he added.
The major cities of Herat, Lashkar Gah, and Kandahar have fallen in the past 24 hours and, while Kabul is larger than any of them, Mr Clarke said: "The momentum is so ferociously with the Taliban at the moment and the Afghan government has done nothing – they've stayed frozen in incompetent corruption, which is what they do.
"So we must expect that Kabul might actually be under threat within a couple of weeks, as opposed to three or four months."
Watch: Timeline of British forces in Afghanistan.
He added: "The Taliban have co-operated with the international community quite a lot so far – they've honoured their agreement not to attack international forces while they're withdrawing.
"I would imagine the Taliban might actually prefer to let the foreign elements get out easily because it makes it easier for them to then pose as the legitimate government of Afghanistan if they haven't left any great legacy issues with the Europeans or the US as they withdraw.
"There is a brutality about what they do, but at the top level, to a degree that has surprised us, they are strategically more sophisticated than most of us thought they would be at this stage."
Mr Clarke said that the future for Afghanistan is likely to involve a long period of civil war – not between the government forces and the Taliban, but between the warlords and the Taliban.
He said the warlords had gathered their forces but had not mobilised yet, meaning the Taliban had not come up against them.
"One suspects there will be a big pushback – not by the Afghan government, which looks like it's in a state of near-collapse, but by the warlords.
"So what Afghanistan is now facing is not so much a united government under the Taliban, but yet more civil war between warlords and Taliban.
"The Taliban are dominant and they'll continue to be dominant, but they never did control the whole of the country – they never controlled the north and the northwest – and chances are they won't again once the warlords start to make war on the Taliban."