The “vast bulk” of British citizens in Afghanistan will be brought back to the UK over the “next few days”, Boris Johnson has said.

Efforts will also be stepped up to relocate Afghans to the UK who have assisted the military operation over the past 20 years, the prime minister said.

The prime minister spoke after chairing an emergency Cobra meeting this afternoon over the deteriorating security situation in Afghanistan, with the Taliban making rapid gains.

Mr Johnson also denied the sacrifices of the British forces over the last two decades have been in vain.

“I don’t think that it was in vain. If you look back at what has happened over the last 20 years there was a massive effort to deal with a particular problem that everybody will remember after 9/11,” he said.

“That was successful. To a very large extent the threat from al Qaeda on the streets of our capital, around the UK, around the whole of the West was greatly, greatly reduced.

“I believe it was right, it was worth it and what we must do now is not turn our backs on Afghanistan.”

He also defended the UK’s role in Afghanistan, saying Britain could be “extremely proud” – but also said countries need to be “realistic” about imposing a military solution.

On the evacuations, the PM said: “In the course of the next few days we will see the vast bulk of UK embassy staff, officials, come back, and step up our efforts to bring back those Afghans who have helped us, the UK, international forces, throughout the last 20 years.”

“To help them, we’re sending out another team of Home Office officials to help them with their applications and get them out.”

He added: “We are appealing to those who have association with the UK who want to come out now to come forward and identify themselves.”

Amid condemnation from some quarters about the US and UK’s decision to withdraw, Mr Johnson said it was a “very difficult” situation but defended the move.

“The UK can be extremely proud of what has been done in Afghanistan over the last 20 years,” he said.

He added: “Thanks to the efforts of the UK armed services, all the sacrifices they made, we have seen no al Qaeda attacks against the West for a very long time… and there are millions of girls and young women who have been educated in Afghanistan.”

The PM added: “I think we have got to be realistic about the power of the UK or any power to impose a military solution – a combat solution – in Afghanistan.

“What we certainly can do is work with all our partners in the region around the world who share an interest with us in preventing Afghanistan once again becoming a breeding ground for terror.”

It comes after the government announced on Thursday that it would deploy 600 UK troops to Afghanistan to help evacuate Britons.

The US has also announced that it is deploying some 3,000 additional troops to help the departure of its embassy staff.

The American embassy will remain open, although personnel will be reduced to a “core diplomatic presence”.

Earlier today, Taliban fighters took control of the Afghan city of Lashkar Gah, following their capture of Kandahar and Herat.

US intelligence has warned that Kabul, the capital, could fall within 90 days.

The US and UK started to withdraw troops from Afghanistan at the end of June and since then the Taliban have gained important strongholds.

Sky’s chief correspondent Stuart Ramsay, who is in Kabul, has said there are unconfirmed reports that the Taliban have captured Logar province, just 40 or 50 miles from the capital.

The capture of Kandahar was the biggest prize yet for the Taliban, which has now taken 12 of Afghanistan’s 34 provincial capitals.

Officials said Kandahar fell on Wednesday night and that government officials and their entourage managed to flee to the airport to escape the city by air.

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As the Taliban draws closer to Kabul, British forces will help the effort to evacuate Britons still in the country.

The number of staff working at the British Embassy in Kabul has also been significantly reduced to a “core group”, the government announced – the remaining staff will focus on consular help to anyone wanting to leave the country.

The British ambassador to Afghanistan, Sir Laurie Bristow, will remain in Kabul but will relocate to a more secure location.