Britain’s evacuation mission from Sudan may have been bought more time after the rival forces agreed a three-day extension to their ceasefire.

The RAF has airlifted nearly 900 people from near the capital Khartoum as they raced to evacuate British nationals ahead of midnight, when the resumption of fierce fighting loomed.

Flights were scheduled to continue regardless but intensified clashes would have added extra pressure to the operation and Foreign Secretary James Cleverly said the mission could become “impossible”.

However, the paramilitary Rapid Support Forces joined the Sudanese military in agreeing to extend the 72-hour truce that has seen fighting lull to allow citizens and foreign nationals to flee.

Britain had been urging the rival generals to extend the ceasefire, which should help stave off a feared humanitarian crisis in Africa’s third largest country.

More than 2,000 British nationals in Sudan have registered under the evacuation plans but the true number of citizens there could be far higher.

As of 4pm on Thursday, the Foreign Office said that 897 people had been evacuated over eight RAF airlifts.

The Foreign Office, which has not said how many of the evacuees are Britons or foreign nationals, had been urging citizens to head to the airstrip before the ceasefire ended.

The flights were however scheduled to run past midnight and around 1,000 people are expected to have been lifted to safety by Friday morning.

Military chiefs say they have the capacity to fly at least 500 people per day out of the Wadi Saeedna airfield, north of Khartoum.

Mr Cleverly warned that resumption of fighting could jeopardise the evacuation efforts.

“We cannot predict exactly what will happen when that ceasefire ends, but what we do know is it will be much, much harder, potentially impossible,” he told Sky News.

“So, what we’re saying to British nationals is if you’re hesitant, if you’re weighing up your options, our strong, strong advice is to go through Wadi Saeedna whilst the ceasefire is up and running.

“There are planes, there is capacity, we will lift you out. I’m not able to make those same assurances once a ceasefire has ended.”

UK passport holders have been told to make their own way to the airfield and are not being provided with a military escort.

Downing Street rejected calls from people, including Foreign Affairs Committee chairwoman Alicia Keans, to widen the eligibility for evacuation beyond British passport holders and their immediate family.

But the Prime Minister’s official spokesman said: “There is an element of discretion for people on the ground as you might expect, given the circumstances and the challenging situation people will be facing.

“We recognise these are very challenging circumstances and, as we have done on previous occasions, we obviously empower people on the ground to make decisions.”

British nationals about to board an RAF aircraft at RAF Akrotiri in Cyprus on Wednesday after being evacuated from Sudan
British nationals about to board an RAF aircraft at RAF Akrotiri in Cyprus on Wednesday after being evacuated from Sudan (Ministry of Defence/PA)

British ambassador to Sudan Giles Lever had been speaking to the warring parties – the Sudanese Armed Forces and the Rapid Support Forces – to try to get a ceasefire extension.

The diplomat, who was not in Sudan when the fighting broke out, was being relocated from London to Addis Ababa in Ethiopia to play a key role in efforts to end the fighting.

Africa minister Andrew Mitchell warned that an end to the ceasefire could result in a humanitarian catastrophe in Sudan.

“It is essential that a ceasefire is maintained and that a political process is secured,” he told the foreign affairs think tank Chatham House.

“If not, the humanitarian consequences will be incalculable.”