Evacuation efforts in Sudan will be “potentially impossible” once a ceasefire ends, the Foreign Secretary said.
James Cleverly said there is no guarantee of further evacuation flights once an agreed halt in fighting expires on Thursday night.
It comes as the Government faces domestic and international criticism over its response.
The British evacuation mission from the African country has seen 536 people taken to safety on six flights so far, according to the latest official figures.
“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 said.
“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.”
More than 2,000 British nationals in Sudan have registered with the Foreign, Commonwealth & Development Office under evacuation plans, but thousands more could be in the war-torn nation.
Defending the pace of the UK response, Mr Cleverly pointed to complications in getting British nationals out of the conflict zone.
“Their nationals in countries don’t all behave the same way.
“Countries where their nationals tend to live in a close expat community who are geographically co-located, it’s easier for them to move en masse, it’s easier for them to be evacuated,” he told BBC Radio 4’s Today programme.
There are concerns the evacuation efforts have seen families split up or some members left behind.
Only British passport holders and immediate family members with existing UK entry clearance are being told they are eligible for evacuation.
Alicia Kearns, the Tory chairwoman of the Foreign Affairs Select Committee, has called for elderly people dependent on children who are British citizens to also be accepted.
“We have given advice as to the status or the prioritisation of the people that we are able to withdraw. We have said it’s British nationals and Sudanese with travel documents.”
Mr Cleverly said some Sudanese nationals have been prevented by the military from reaching the airstrip – but spoke of “difficult cases”.
“Of course, we are trying to facilitate as many people to leave the country as possible,” he said.
Downing Street said that evacuation eligibility requirements had not changed but that there was “an element of discretion” for those working on the ground in Sudan.
“You should not go to the airport unless you are a British passport holder or their dependent, that is set out very clearly.
“I think what we are saying is 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.”
Mr Cleverly also said he does not recognise reports that the UK caused delays in Germany’s efforts to evacuate its citizens from Sudan with its mission to rescue British embassy staff at the weekend.
The BBC said it was told by senior German political sources that British forces landed in the country without the permission of the Sudanese army, angering them so much that they temporarily barred access to the airfield that European nations were hoping to use.
“My understanding is we did… we did have permissions for those overflights,” he said.
Downing Street said there were no current plans to create a specific resettlement scheme for anyone fleeing Sudan.
Earlier, Mr Cleverly pointed to the fact there is “war and conflict all over the world”.
“There are literally millions upon millions of people who are in countries plagued by war.
“We recognise that we cannot host everybody who is in a country plagued by war.”
Africa Minister Andrew Mitchell also warned that an end to the ceasefire could result in a humanitarian catastrophe in Sudan.
He told the foreign affairs think tank Chatham House: “It is essential that a ceasefire is maintained and that a political process is secured.
“If not, the humanitarian consequences will be incalculable.”