Thousands of British nationals in Sudan have been warned that there is no guarantee on further evacuation flights once the ceasefire expires on Thursday night.
Military chiefs said at least 500 people a day can be rescued from an airfield near the capital Khartoum, amid fears bloody clashes will resume when the fragile truce between the warring factions ends.
Foreign Secretary James Cleverly warned the UK “cannot guarantee” how many further flights will depart after the deadline is reached at midnight local time.
He urged UK nationals who wish to leave Sudan to make their way to the Wadi Saeedna air strip “as soon as possible”.
More than 530 people had been airlifted to safety on six flights as of 9pm on Wednesday, the Foreign, Commonwealth and Development Office (FCDO) said.
The first plane carrying UK citizens touched down on British soil earlier in the day, having left Larnaca in Cyprus, where they were initially evacuated.
More than 2,000 British nationals in Sudan have registered with the FCDO under evacuation plans, but thousands more could be in the war-torn nation.
Prime Minister Rishi Sunak was told by military chiefs that flights can continue “for as long as we need to” even if the US-brokered pause in fighting breaks.
Mr Cleverly tweeted: “The 72-hour ceasefire in Sudan ends tomorrow night (April 27).
“We cannot guarantee how many further flights will depart once the ceasefire ends.
“If you plan to leave Sudan please travel to the British Evacuation Centre as soon as possible.”
But he warned “we are absolutely in the hands of the ceasefire”.
Mr Mitchell told Sky News: “We are doing everything we can to make sure it’s prolonged and on the wider stage, too, trying to negotiate for a longer ceasefire, because if the combatants don’t lay down their arms and return to barracks, there’s going to be a humanitarian catastrophe in Sudan.”
He said that “at the moment those safe and legal routes don’t exist” for refugees from Sudan to claim asylum in the UK.
Only British passport holders and immediate family members with existing UK entry clearance are being told they are eligible for evacuation.
But Alicia Kearns, the Tory chair of the Foreign Affairs Select Committee, said elderly people dependent on children who are British citizens should also be admitted.
She told the BBC: “In the same way we treat children who are dependent on their parents, we should respect that some elderly people are dependent on their children.
“So, I think it is important that we are bringing people out who would otherwise be left destitute and really vulnerable.”
The BBC reported that it was told by senior German political sources that British forces landed in the country without the permission of the Sudanese army, so angering them that they temporarily barred access to the airfield that European nations were hoping to use.
However, a defence source dismissed the claim the UK military arrived without authorisation as “complete nonsense”, while the Ministry of Defence denied that Britain was responsible for any delay.
The Government is working to provide other routes out of Sudan, with HMS Lancaster progressing towards Port Sudan, the Red Sea dock some 500 miles from Khartoum that could possibly be used in a seaborne operation.
But with UK nationals being told to make their own way to evacuation sites, that trip would be made particularly challenging by fuel shortages and the traffic of people fleeing.
Women with small children and babies in prams, as well as elderly individuals in wheelchairs were among those met by family members and friends waiting anxiously in the arrivals lounge.
One man from Sudan, who did not give his name, described his experience as a “nightmare”.
Speaking outside the airport, he said: “It’s absolutely fantastic to be back. It’s been a nightmare. We’ve never seen anything like it before.
“We saw it on the television before but we never thought it was going to happen to a peaceful country like Sudan.
“Khartoum is like a ghost city, everyone is leaving Khartoum now.
“We are very grateful to the British servicemen and women who risked their lives to come to Sudan and help us out.”