The former cabinet secretary Sir Mark Sedwill said the decision to withdraw from Afghanistan was “a bad policy, badly implemented”, which runs the risk of leading to a significant refugee crisis following the Taliban’s takeover of the country.

Sedwill, who was Britain’s most powerful civil servant until last September, warned that the UK and its allies “do not have a coherent policy plan in place” to deal with any mass overland migration now that the Kabul airlift is over.

Speaking at a Policy Exchange thinktank event, Sedwill said that while the US decision to exit Afghanistan after 20 years could not be changed, it had a series of important consequences that required action from Britain and the west.

“First, there will need to be a major humanitarian effort in and around Afghanistan. We will be very lucky indeed if there is not a really significant refugee crisis,” the former mandarin said, unless the Taliban run an “inclusive and wholly different government” entirely unlike their previous period of rule before 2001.

While the evacuation operation, in which 114,000 people were airlifted out of Kabul in just over two weeks, was a “tactical effective success”, Sedwill said, its achievement “can’t and shouldn’t conceal that overall, we do not yet have a policy and plan in place to deal with refugee flows out of Afghanistan”.

Thousands of people recognised as having a firm or likely claim for resettlement in the UK were left behind in Afghanistan when the airlift ended – leaving Britain to call directly on the Taliban to allow others safe passage out of the country.

Last week Ben Wallace, the defence secretary, suggested that Afghans hoping to be evacuated might be better off heading to one of the country’s land borders, where Britain would accept claims made from neighbouring countries, particularly regarding Afghan translators and others who worked directly for the UK.

Britain and other western countries have also been calling on the Taliban to allow safe passage for those who want to leave the country, with the UK one of a number of countries engaged in direct talks with the insurgent group’s political leadership, who are based in Qatar.

But Sedwill warned that the continuing goodwill of neighbouring countries such as Pakistan could not necessarily be relied on given the large numbers of Afghans who have fled their country following years of conflict. “The neighbours really can’t absorb more,” he warned.

There were already an estimated 2.2 million Afghan refugees in neighbouring countries, according to UN figures, before the latest crisis that has caused tens of thousands to head to the borders as the Taliban seized control. The UNHCR estimates that in a worst-case scenario, as many as 500,000 more refugees could flee the country to Pakistan, Tajikistan, Iran, Uzbekistan and Turkmenistan in the coming months.

Tackling the movement of people would require a greater effort than the two-week airlift, Sedwill said, which dealt with “relatively small numbers”.

Sedwill was the UK’s cabinet secretary and national security adviser from April 2018 to September 2020 and served in roles in Afghanistan a decade ago. But he was forced out by the prime minister, Boris Johnson, with a promise that the UK would put him forward for the job of head of Nato in the future.