An attack from ISIS could happen in Kabul within "hours", according to Armed Forces Minister James Heappey.
The stark warning comes as the Government changed travel advice to and from Afghanistan on Wednesday night.
Speaking to LBC on Thursday morning, Mr Heappey was asked whether the imminence of an attack could be measured in days.
He replied bluntly: "Hours. Hence the urgency of changing the travel advice last night.
"I was given lines today for what might happen if the attack happened while I was doing this media round."
Mr Heappey added: "I don't think everybody should be surprised by this. Daesh, or Islamic State, are guilty of all sorts of evil.
"But the opportunism of wanting to target a major international humanitarian mission is just utterly deplorable but sadly true to form for an organisation as barbarous as Daesh," he added.
As the US, UK and other NATO allies have continued the evacuation of Kabul airport in recent days, a threat is said to have emerged from the less well-known terrorist group, ISIS-K.
Watch: Who are ISIS-K?
On Tuesday, US President Joe Biden said "every day [the US are] on the ground" in Afghanistan "is another day we know that ISIS-K is seeking to target the airport, attack both US and allied forces, and innocent civilians".
Sajjan Gohel, the international security director at the Asia-Pacific Foundation think-tank, told Forces News the group "have fought with the Taliban in the past, but because of tribal and lineage connections they actually also sometimes co-operate with the Taliban".
He also said the group was a threat both inside Afghanistan "and, potentially, if left unchecked," internationally.
If the 31 August withdrawal deadline remains in place, British troops will have to leave before the US and hand over tasks and responsibilities to other nations.