Boris Johnson is to convene an emergency Cobra meeting this afternoon over the worsening situation in Afghanistan.
Taliban fighters took control of the Afghan city of Lashkar Gah on Friday, following their capture of Kandahar and Herat, as they continue to make advances across the country.
With weeks to go before the US plans to withdraw the last of its troops, the insurgents now control more than two-thirds of Afghanistan.
US intelligence has warned that Kabul, the capital, could fall within 90 days.
A No 10 spokesperson announced the meeting this afternoon, saying: “The prime minister is convening a COBR this afternoon to discuss the current situation in Afghanistan.”
On Thursday it was announced that 600 UK troops would be sent to Afghanistan to help evacuate Britons.
It is almost 20 years since the invasion of Afghanistan was launched in the wake of the 9/11 attacks to oust the Taliban and prevent it from harbouring al Qaeda, the group behind the 2001 terror attack on the US.
Since the initial invasion, the US and its NATO allies have spent almost two decades trying to establish a functioning state.
A total of 456 British forces personnel or MoD civilians died in Afghanistan.
But with the 20th anniversary of 9/11 looming, the Taliban has seized more than a dozen cities.
Speaking to Sky News earlier, the defence secretary said terror group al Qaeda “will probably come back” in Afghanistan as the security situation continues to deteriorate.
Ben Wallace told Kay Burley: “I’m absolutely worried that failed states are breeding grounds for those types of people.
“Of course I am worried, it is why I said I felt this was not the right time or decision to make because, of course, al Qaeda will probably come back, certainly would like that type of breeding ground.
“That is what we see, failed states around the world lead to instability, lead to a security threat to us and our interests.”
The defence secretary said it was his view that the deal signed by the US and the Taliban in Qatar in 2020 under then-Republican president Donald Trump to withdraw from Afghanistan was a “mistake” and described it as a “rotten deal”.
Mr Trump’s successor, Democrat Joe Biden, has continued with the withdrawal timetable since taking office in January.
Mr Wallace said of the deal: “I felt that that was a mistake to have done it that way, that we will all as an international community pay the consequences of that, but when the United States as the framework nation took that decision, the way we were all configured, the way we had gone in meant that we had to leave as well.”
Speaking to Sky News, former international development secretary Rory Stewart described the situation in Afghanistan as “terrifying” and added: “It’s a total betrayal by the United States and by the United Kingdom.”
He said it was not a given that other countries had to pull out once the US had decided to do so, expressing his view that NATO countries could have “taken up that slack”.
And Mr Stewart said “we are going to end up with terrorists” as a result of what is going on.
“This is a horrifying group associated with terrorists, they have been backing suicide bombing in the areas they control, women are not going to school and it is a total betrayal by the United States and the United Kingdom,” he continued.
Mr Stewart added that Britain and the US would have to “expect to take a lot of refugees” given what is happening, because “this is our fault”.
Conservative MP Johnny Mercer, a former British Army officer who completed three tours in Afghanistan, said the situation in the country was a “tragedy”.
“I think the way the international community, but particularly the UK and the US, have pulled the rug from the Afghan security forces in this way is humiliating,” he told Kay Burley.
Labour’s shadow foreign secretary Lisa Nandy said the progress that has been made in Afghanistan in recent years was “unravelling in front of our eyes”.
Speaking to BBC’s World at One programme, she called on the government to help find a “political solution” to the crisis, as well as “reinstating the aid that has been cut to people in Afghanistan, supporting refugees by having legal routes for people to get out if they need to, and also to be working with our NATO partners to co-ordinate a proper response”.