Significant tensions have erupted over intelligence failures between the Foreign Office and the Ministry of Defence, fuelled by comments made by the defence secretary, Ben Wallace. In an interview on Thursday, Wallace contrasted his department’s handling of the Afghanistan crisis with that of Raab’s embattled Foreign, Commonwealth and Development Office.

Raab told MPs that he had believed the Afghan capital would remain safe until next year, backed by the prevailing intelligence assessment. Wallace said he first thought “the game was up” in Afghanistan and the western-backed government would fall “back in July”.

Speaking on Sunday, Carter, the chief of the defence staff, said: “It was entirely possible that the government wouldn’t hold on that much longer. Indeed, many of the assessments suggested it wouldn’t last the course of the year and, of course, that’s proven to be correct.”

Carter said that “everybody got it wrong” on the Taliban’s strategy. “There’s been a lot of talk about a failure of intelligence and all of that; the plain fact is, and I said to you on that programme when you interviewed me on 11 July, that there are a number of scenarios that could play out and one of them certainly would be a collapse and state fracture,” he told the BBC’s The Andrew Marr Show.

“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.”

Carter also said the Taliban would need to deal with their “catastrophic success” and would struggle to run the country, adding: “If they behave, perhaps they will get some help.”

He said: “If the political commission is able to form an inclusive government, it is possible that they may govern less repressively – we have to wait and see.

“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.

“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.”

Approval ratings for Raab have plunged among party members amid damaging stories about his return from holiday to handle the crisis and an anticipated reshuffle.

A survey by Conservative Home showed Raab dropping 21 places in the cabinet approval league table, from third-most popular to fourth from bottom. The site, which regularly surveys party members, said the fall was one of its biggest ever, similar to Theresa May’s plummeting approval rating after the 2017 election.