"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," he added, echoing similar warnings by other Western nations like the United States.

He said later that the attack could come within hours.

Heappey said that "thousands of people" had chosen to keep waiting outside the airport regardless, desperate to board evacuation flights after the Taliban takeover of Afghanistan.

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.

Prime Minister Boris Johnson told journalists: "We have to be realistic about what's going on, and you'll appreciate that there are Islamic State Khorasan (the regional branch of IS) out there."

While meeting military personnel coordinating the evacuation in London, Johnson said the "overwhelming majority" of those eligible have been evacuated.

He acknowledged the time remaining may be "quite short".

Around 15,000 people have been evacuated by British troops, the prime minister said.

The threat warning came as the UK was planning 11 flights out of Kabul airport in the next 24 hours, Heappey said.

Late Wednesday, Britain warned its citizens away from Kabul's airport.

"Move away to a safe location and await further advice," the Foreign and Commonwealth Office wrote on its website, adding that "if you can leave Afghanistan safely by other means, you should do so immediately".

Among those waiting outside the airport was Paul "Pen" Farthing, a former Marine who runs an animal shelter in Kabul, and has chartered a plane to fly out his staff's families and animals, prompting controversy as thousands of desperate Afghans seek to flee the Taliban regime.

He tweeted saying his team and animals are stuck outside the airport and requesting the Taliban allow safe passage for his convoy.

Heappey told Sky News that Farthing's request for safe passage for his Afghan staff and animals was another way of asking to be "brought to the front of the queue".

"That doesn't feel like the right thing to do," the minister said.