The final stages of the evacuation effort in Afghanistan are under further strain after a warning that a “highly lethal” terror attack could be launched within hours.

Armed forces minister James Heappey said on Thursday there is “very credible reporting” of an “imminent” and “severe” threat to Kabul airport.

He called on those queuing outside Hamid Karzai International Airport to move to safety amid concerns over an affiliate of the so-called Islamic State in Afghanistan, know as Isis-K.

The threat is heaping extra pressure on the operation to help people flee the nation captured by the Taliban, with Tuesday’s deadline for foreign troops to leave fast approaching.

Mr Heappey told BBC Breakfast: “The credibility of the reporting has reached the stage where we believe there is a very imminent, a highly lethal, attack, possible within Kabul.

“And, as a consequence, we’ve had to change the travel advice to advise people not to come to the airport, indeed to move away from the airport, find a place of safety and await further instruction.”

He told LBC radio the possible attack could come within “hours”, adding: “I don’t think anybody 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.”

The defence minister acknowledged the “extraordinarily challenging” decision people trying to flee are facing because they also fear for their lives by leaving the queue and potentially having to remain in Afghanistan.

“And so I think there is an appetite by many in the queue to take their chances, but the reporting of this threat is very credible indeed and there is a real imminence to it,” he told BBC Radio 4’s Today programme.

The warning to stay away from Kabul airport is a stark change in tone from a week ago when Boris Johnson said the situation had been stabilising.

Eight RAF flights managed to lift 1,988 people from Kabul within the past 24 hours, Mr Heappey said, taking the total since the Taliban began its march to power to 12,279.

He said Britain has 11 more flights scheduled out of Kabul for Thursday but declined to say whether that will be the end of the operation, citing the security of troops on the ground.

But its climax is rapidly approaching after US President Joe Biden rejected calls from the Prime Minister and other allies to delay his withdrawal date for the remaining US troops.

They are providing security at Kabul airport, meaning other allied forces are expected to have to wind down their evacuation efforts and depart ahead of the Americans.

With the main route to flee under threat, Defence Secretary Ben Wallace reportedly told MPs that crossing the Afghanistan border in order to leave would be a “better option”.

The Government has previously said it will increase diplomatic support in neighbouring countries to process refugees who escape from Afghanistan.

On Wednesday, it was believed nearly 2,000 people assessed as eligible under the Afghan relocations and assistance policy (Arap) remained on the ground.

But Mr Heappey said the number outstanding is now “potentially half” of the previous estimate.