Boris Johnson has said is it an "illusion" to think Britain alone could have prevented the collapse of Afghanistan to the Taliban after the US withdrew its forces.
Speaking at an emergency sitting of parliament, the prime minister denied the government had been unprepared for the Taliban takeover at the weekend.
He told a packed House of Commons that he is now working to evacuate the remaining British nationals and their allies stuck in Afghanistan.
The Government has faced intense criticism – not least from its own backbenchers – following the rapid unravelling at the weekend of the Western-backed government of President Ashraf Ghani in the face of the Taliban advance.
Mr Johnson said when ministers came to consider the UK’s options after the US announced its intention to withdraw, they came up against the “hard reality” that there was no will among allies to continue without the Americans.
British citizens and dual nationals residing in Afghanistan board a military plane for evacuation from Kabul airport, Afghanistan August 16, 2021.
UK MOD Crown copyright 2021
“The West could not continue this US-led mission, a mission conceived and executed in support of America,” he said.
“I really think that it is an illusion to believe that there is appetite amongst any of our partners for a continued military presence or for a military solution imposed by Nato in Afghanistan. That idea ended with the combat mission in 2014.
“I do not believe that today deploying tens of thousands of British troops to fight the Taliban is an option that, no matter how sincerely people may advocate it – and I appreciate their sincerity – but I do not believe that that is an option that would commend itself either to the British people or to this House.
“We must deal with the position as it is now, accepting what we have achieved and what we have not achieved.” Labour leader Sir Keir Starmer said “it’s been a disastrous week, an unfolding tragedy” in Afghanistan, and added the prime minister must "snap out of his complacency".
Sir Keir told the Commons: "The Defence Secretary has said that some people who worked with us will not get back. Unconscionable. The Government must outline a plan to work with our allies to do everything that’s possible to ensure that does not happen.
“There are reports this morning from NGOs that an evacuation plane left almost empty this morning because evacuees couldn’t get to the airport to get to that plane.”
He added: “We do not turn our backs on friends at their time of need. We owe an obligation for the people of Afghanistan.
"There should be a resettlement scheme for people to rebuild their lives here. Safe and legal routes, it must be a resettlement scheme that meets the scale of the enormous challenge, but what the Government has announced this morning does not do that.”
Former prime minister Theresa May warned of a potential humanitarian crisis, as thousands of Afghans try to flee the country.
"We must be deeply concerned about the possible impact here in the UK," she said.
"The aim of our involvement in Afghanistan was to ensure that it could not be used as a haven for terrorists. Terrorists who could train, plot and encourage attacks here in the UK.
“Al Qaeda has not gone away, Daesh (also known as the so-called Islamic State) may have lost their ground in Syria, but these terrorist groups remain and they have spawned others. We will not defeat them until we have defeated the ideology which feeds their extremism.”
She added: “The Taliban of course have said that they will not allow Afghanistan to become a haven for terrorists again… their actions must be what we look at, not their words and their action has…. been to release thousands of high-value Taliban, al Qaeda and Daesh fighters.”
“So their action is completely different from their words and I think it is absolutely essential for us to recognise that the probability that Afghanistan will once again become a breeding ground for the terrorists who seek to destroy our way of life.”
