Prime Minister Boris Johnson has said the UK departure from Afghanistan was “the culmination of a mission unlike anything we’ve seen in our lifetimes”.

The final UK troops and diplomatic staff were airlifted from Kabul on Saturday, drawing to a close Britain’s 20-year engagement in Afghanistan and a two-week operation to rescue UK nationals and Afghan allies.

Ambassador to Afghanistan Sir Laurie Bristow, who had been processing those fleeing the country at the airport until the last moment, was among those who landed at RAF Brize Norton base in Oxfordshire on Sunday morning.

And it brought to a close Operation Pitting, believed to be the largest evacuation mission since the Second World War.

In a video uploaded to Twitter on Sunday morning, Mr Johnson praised the more than 1,000 military personnel, diplomats and officials who took part in the operation in Afghanistan.

He said: “UK troops and officials have worked around the clock to a remorseless deadline in harrowing conditions.

“They have expended all the patience and care and thought they possess to help people in fear for their lives.

“They’ve seen at first-hand barbaric terrorist attacks on the queues of people they were trying to comfort, as well as on our American friends.

“They didn’t flinch. They kept calm. They got on with the job.

“It’s thanks to their colossal exertions that this country has now processed, checked, vetted and airlifted more than 15,000 people to safety in less than two weeks.”

Shortly afterwards, Sir Laurie arrived on one of the last flights carrying UK military and civilian personnel on their final homeward leg back from Afghanistan.

A Voyager aircraft touched down at RAF Brize Norton airfield in Oxfordshire on Sunday morning carrying roughly 250 personnel on board, including members of 16 Air Assault Brigade who were stationed at Kabul airport.

The plane flew in from Al Minhad airfield in the United Arab Emirates near Dubai, where the UK’s evacuation flights from Afghanistan first landed.

Further flights carrying personnel are expected later on Sunday.

Speaking on the runway, Sir Laurie vowed to continue to help British nationals and Afghans who remain in the country and still need help.

“We’ve had to leave Afghanistan for now and the embassy will operate from Qatar for the time being,” he said.

“We will continue to stand by the people of Afghanistan, working on humanitarian, diplomatic and security work, and above all bringing to the UK Afghans and British nationals who still need our support, and we will be putting pressure on the Taliban to allow safe passage for those people.

“We will reopen the embassy as soon as we can. We will do everything we can to protect the gains of the last 20 years and above all to help the Afghan people achieve the security and the peace that they deserve.”

He also said: “It’s been an extraordinary, intense effort by the Foreign Office, the military and Border Force together to bring over 15,000 people to safety in under two weeks.”

The Government said of the 15,000 people evacuated since the Taliban seized Kabul, 5,000 of those were British nationals and their families.

More than 8,000 Afghans who helped the British effort as interpreters or in other roles, or who are otherwise vulnerable to persecution by the regime, were also able to flee to safety with their families.

But Vice Admiral Ben Key, Chief of Joint Operations, who commands Operation Pitting, admitted there was a “sense of sadness” that not all could be saved.

Speaking at RAF Brize Norton on Sunday morning, he said: “Whilst we recognise and I pay testament to the achievement of everything that has been achieved by coalition forces, but particularly the British contingent, over the last two weeks, in the end we know that there are some really sad stories of people who have desperately tried to leave that we have – no matter how hard our efforts – we have been unsuccessful in evacuating.”

He added: “There has been a phenomenal effort achieved in the last two weeks. And I think we always knew that somewhere we would fall just short.

“So, this isn’t a moment of celebration for us at all, this is a moment to mark a tremendous international effort to evacuate as many people as we could in the time available.”

But former head of the British Army General Lord Richard Dannatt suggested more could have been done if the Government had not been “asleep on watch”.

Speaking to Times Radio, he said: “It is unfathomable why it would appear that the Government was asleep on watch.

“I think the issue of Afghanistan sat on the backburner. Maybe it started to come forward. But then, suddenly, when the Taliban took over the country in the precipitate fashion in which they did, it fell off the cooker straight on to the kitchen floor and we’ve … had this chaotic extraction.”

Addressing the families and loved ones of the British troops who “gave their all”, Mr Johnson said: “Your suffering and your hardship were not in vain.”

He added: “It was no accident that there’s been no terrorist attack launched against Britain or any other western country from Afghanistan in the last 20 years.”

We lacked the strategy, the statecraft, the patience to see through, and the manner of our departure is a humiliation, a confirmation of our diminished resolve

But Conservative MP and veteran Tobias Ellwood said the UK had “very little to show” for 20 years in Afghanistan.

The chairman of the Commons Defence Select Committee told LBC: “Our armed forces performed so valiantly but they were let down by their political masters.

“We lacked the strategy, the statecraft, the patience to see through, and the manner of our departure is a humiliation, a confirmation of our diminished resolve, and our adversaries will not be slow to exploit it.”