The last dedicated evacuation flight back to the UK left Kabul overnight, Sky News understands.

Remaining flights over the weekend are set to bring home British troops as well as the final small numbers of Afghan evacuees who have permission to fly, defence sources said.

The UK ambassador in Kabul, Laurie Bristow, said in a video posted on social media that his team had been “working until the very last moment”.

Almost 15,000 people have been airlifted to safety, he added, tweeting that “our commitment to the people of Afghanistan will endure”.

Nearly 15,000 British nationals, Afghan staff and others at risk have been evacuated from Kabul since Operation Pitting began – our commitment to the people of Afghanistan will endure. pic.twitter.com/zUQ52ps1cE

Earlier, the chief of the defence staff, General Sir Nick Carter, said the final stages of the withdrawal and evacuation represented the “hardest” of any phase of war.

He added: “There has to be an ever diminishing rearguard that protects your back as you’re evacuating yourselves, and of course there comes a point when it really is the last aeroplane out.

“There are all sorts of things that can go wrong” when the final planes leave, such as the “threat of terrorism”, he said.

“You are at your very most vulnerable at the point at which your last aeroplane is waiting to lift.”

Sir Nick warned Britain is “not out of the woods yet”, adding: “The operation will carry on for a little bit longer. But it’s been a huge enterprise.”

Troops are working in the face of a “very demanding threat” while also focusing on the “humanitarian job” with soldiers “holding a gun in one hand and a baby in the other”.

“It has been very difficult,” Sir Nick said.

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“The plain fact is we have always got that in the back of our minds.

“For the troops on the ground, they have to be constantly alert and constantly thinking about how they can rebut the threat.

“I think we should be holding our breath and thinking very hard about that last airplane or several last airplane-loads and thinking about what a challenge it is going to be for those very brave people who are trying to effect their departure in as safe and orderly fashion as they can.”

Sir Nick, who spent almost three years in Afghanistan, described his regret that the 20-year mission there had ended with the Taliban in charge and a grave terrorist threat.

He said of the withdrawal: “It’s not how we hoped it would end, that’s absolutely right.”

He added: “I think we have done an extraordinary job to evacuate as many as we can but I’m afraid it’s absolutely heart-breaking we can’t get everybody out.

“If they are able to get out they will always be welcome but the awful fact of life is difficult decisions have to be made.”

He told of his heartbreak that Britain was unable to rescue all Afghans with the right to travel to the UK, adding: “Not a day passes without me having a bit of a tear in my eye.”

US forces, which have by far the largest presence, will be the last to depart ahead of a deadline of Tuesday, set by President Joe Biden.

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