LONDON, (Reuters) - Prime Minister Boris Johnson defended Britain's airlift from Kabul on Sunday following growing criticism that ministers had been "asleep on watch", potentially leaving thousands of eligible Afghans behind in the country.

Britain, like the United States, has been criticised for failing to predict how quickly the Afghan government would fall, meaning it had not made sufficient preparation for the chaos that would follow when the Taliban seized power.

Britain's last military flight left Kabul late on Saturday, ending a chaotic two weeks in which soldiers helped to evacuate more than 15,000 people from the crowds who descended on the capital's airport, desperate to flee the country.

Richard Dannatt, former chief of staff of the British army, said the government now needed an inquiry to establish why it had been so ill-prepared for the rapid turn of events.

"It is unfathomable why it would appear that the government was asleep on watch," he told Times Radio. "We've had this chaotic extraction, we should have done better, we could have done better."

The Observer newspaper said thousands of emails sent to the Foreign Office by lawmakers and charities, detailing cases of Afghan citizens who needed help to leave, had not been read.

A Foreign Office spokesperson said it had worked tirelessly to evacuate people but it had always cautioned that it would not be able to help everyone. The spokesperson declined to give any further details on the process.

Britain's defence minister, Ben Wallace, predicted last week that time had run out to evacuate around 1,000 Afghans who were eligible to come to Britain, including former staff to the UK.

Lisa Nandy, the opposition spokeswoman for the foreign office, said ministers had been completely unprepared for the withdrawal. "It really is an unparalleled moment of shame for this government, that we've allowed it to come to this," she told Sky News.

Johnson said however that while Britain would not have wished to leave Afghanistan in the way it did, the armed forces should still be proud of their achievements.

Speaking to the 150,000 men and women who completed a tour of Afghanistan, and the families of the 457 who died there, Johnson said they had succeeded in keeping Britain safe and in improving the livelihoods for Afghan locals.

"I thank everyone involved, and I believe they can be very proud of what they've done," he said in a video online.

A flight carrying troops and London's ambassador, Laurie Bristow, landed in Britain on Sunday, with further flights expected. Bristow said Britain's embassy to Afghanistan would operate in Qatar for now, before it can return when safe to do so.