"Reporting over the week has become ever more credible. And it is of an imminent and severe threat to life," Heappey told Times Radio.

"This is a very serious threat, very imminent," he said.

Late Wednesday, Britain warned its citizens away from Kabul's airport, where huge crowds have gathered hoping to board evacuation flights after the Taliban takeover of Afghanistan.

"Do not travel to Kabul Hamid Karzai International Airport," the Foreign and Commonwealth Office wrote on its website. "There is an ongoing and high threat of terrorist attack."

"If you are in the area of the airport, move away to a safe location and await further advice," it urged, adding that "if you can leave Afghanistan safely by other means, you should do so immediately".

Washington and its allies have been flying out thousands of Afghans every day on hulking military transport, but it has become an increasingly difficult and desperate task.

Heappey said his advice to British citizens was "to do as the Foreign Office advice instructs and to find a place of safety."

According to the latest data, 11,474 people have been evacuated by the UK since August 13, including 6,946 Afghans.