Boris Johnson will chair a third Cobra meeting over the worsening situation in Afghanistan as No 10 pledged to continue to evacuate people from the country for as long as it is safe to do so.

The Prime Minister will hold a third Cobra meeting in four days on Monday afternoon as a desperate struggle to get UK nationals and local allies out of the country continues.

His official spokesman said the UK will continue the evacuation effort for “as long as we are able to do so and as long as it is safe to do so” as the Government came under fire for the way in which the crisis has been handled.

But Defence Secretary Ben Wallace, who served in the Scots Guards, appeared to choke up as he spoke of his regret that “some people won’t get back”.

Speaking on LBC, Mr Wallace said: “It’s a really deep part of regret for me… look, some people won’t get back. Some people won’t get back and we will have to do our best in third countries to process those people.”

Asked why he felt the situation “so personally”, Mr Wallace replied: “Because I’m a soldier… because it’s sad and the West has done what it’s done, we have to do our very best to get people out and stand by our obligations and 20 years of sacrifice is what it is.”

British troops are racing against the clock to get people out of Afghanistan following the dramatic fall of the Western-backed government amid a rapid advance across the country by the Taliban.

The Prime Minister’s official spokesman said on Monday there would be “significant numbers flying out day by day” and that the Afghan Relocations and Assistance Policy was “open-ended”.

He said the British ambassador, Sir Laurie Bristow, was working from the airport in Kabul alongside Home Office staff, diplomatic workers, and the armed services, to process visas.

He told a Westminster briefing: “We will continue to do everything we can, our offer is open-ended, we haven’t put an end date on that and we will continue to do all we can including – as the Defence Secretary said – should individuals manage to get to other countries and be brought in from those other countries.”

The Government is coming under sustained pressure over the situation in Afghanistan, with Conservative MPs criticising the handling of the crisis.

Families of those who died fighting in Afghanistan have also condemned the UK and US over the withdrawal.

John Baron, Tory MP for Basildon and Billericay, has called for the PM to apologise to veterans and families who lost loved ones.

Former defence minister Tobias Ellwood, the Tory MP for Bournemouth East, described chaotic scenes at Kabul airport as “Saigon 2.0”, referencing evacuations in 1975 as the North Vietnamese army captured the city and ended the Vietnam War.

In a tweet, he said: “Is this how we thought we’d depart Afghanistan? I repeat my call for a UK inquiry.”

While Nus Ghani, Conservative MP for Wealden, told the BBC: “In one fell swoop we’ve taken the country back 20 years.”

Crossbench peer and former cabinet secretary Lord Sedwill warned this was “a humiliating moment for the West”.

And Graham Knight, father of 25-year-old RAF Sergeant Ben Knight, who was killed when his Nimrod aircraft exploded in Afghanistan in 2006, said: “As for whether people’s lives were lost through a war that wasn’t winnable, I think they were.”

However No 10 defended the Government’s position.

The PM’s spokesman said: “I think it was clear that military intervention alone was not going to be sufficient. We have seen the Taliban move quickly across Afghanistan, that is true, but we have been monitoring the situation, and are continuing to do everything possible to secure UK and Afghan nationals.”

He said: “Clearly, once the US decision was made (to withdraw troops), our view was that it would not be right to act unilaterally in this as an occupying force.

“We did speak to other international partners on this, but it was clear that that wasn’t going to be feasible. So we have focused on doing everything possible to enable… to work with the previous Afghan government and to now facilitate the exit of UK nationals and Afghan nationals.”

He said: “We want to obviously continue to do this as long as we are able to do so and as long as it is safe to do so. You’ll appreciate the US have said that they will be leaving at the end of the month so we will keep that under review and we’ll continue to do it as long as we can do so because we want to get as many people out as we can.”

Labour leader Sir Keir Starmer said there was an “obligation” to those in Afghanistan who had helped the UK effort.

Speaking on a trip to Wolverhampton, he said: “We need to get UK nationals out, but we also have an obligation to all of those Afghans who helped and assisted the UK, and we shouldn’t have nice distinctions between this type of person, this type of help, and that type of help.

“If those in Afghan have helped us, the UK, in our work in Afghanistan, we have got an obligation to them.”