Head of the Armed Forces, General Sir Nick Carter, said many assessments suggested Kabul would fall this year, despite Foreign Secretary Dominic Raab having said intelligence put this as "unlikely".
Boris Johnson and Mr Raab, who was holidaying in Crete in August while the Taliban marched back to power, have been coming under sustained criticism for their handling of the crisis, with thousands of vulnerable Afghans feared to have been left behind.
However, General Sir Nick denied to the BBC on Sunday that the military intelligence was wrong.
"No. The first scenario I think also would've said is it was entirely possible that the government wouldn’t hold on that much longer," he told The Andrew Marr Show.
"Indeed, many of the assessments suggested it wouldn’t last the course of the year and, of course, that’s proven to be correct."
General Sir Nick said "there are a number of scenarios that could play out and one of them certainly would be a collapse and state fracture".
"I think everybody got it wrong," he said.
"It was the pace of it that surprised us and I don't think we realised quite what the Taliban were up to.
"They weren't really fighting for the cities they eventually captured, they were negotiating for them, and I think you'll find a lot of money changed hands as they managed to buy off those who might have fought for them."
General Sir Nick said even the Taliban did not expect to take back power of Afghanistan so swiftly as the US pulled out its troops ahead of the 31 August deadline.
"At the moment they suffer from what we military call catastrophic success.
"They were not expecting to be in government as quickly as they have appeared and the reality is they are trying to find their feet," he said.
"We need to wait and see how this happens and recognise that they're probably going to need a bit of help in order to run a modern state effectively and if they behave perhaps they will get some help."
However, Tom Tugendhat, chairman of the Foreign Affairs Select Committee, said it would be "bizarre" to hold formal talks with the Taliban, given that it doesn't have a formal structure.
The Conservative MP told Sky’s Trevor Phillips on Sunday: "The UK has always had a policy of reaching out in different ways to different groups.
"The fact that it still hasn’t formed a government two, three weeks after it took Kabul is partly an indicator of the divisions within the organisation, it is fighting within itself as to what it does.
Watch: The Prime Minister praises military and 'amazing' Kabul airlift effort.
"So, the idea that these talks are meaningful in a traditional sense, I’m afraid they’re not.
"The idea that the Taliban is changed, it’s got a much slicker PR outfit, I don’t know who it’s hired but it’s got a similar group to the people who are doing ISIS PR in northern Syria, but the reality that it’s changed the way it treats women or murder minorities is complete rubbish, they’re still as brutal and vicious as they were."
More than 8,000 former Afghan staff and their family members were among the 15,000-plus people evacuated by the UK since 13 August as part of Operation Pitting.
But up to 1,100 Afghans deemed eligible were estimated to have been left behind, though that figure will fall short of the true number the UK would wish to help.
Either the Prime Minister or the Foreign Secretary is expected to give a statement to the Commons during the week.
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Cover image: General Sir Nick Carter (Picture: PA/Alamy).