RAF rescue flights plucking British nationals to safety from Sudan are due to start taking off from an airfield in the war-torn nation tonight.
But not all the estimated 4,000 Britons trapped in the African country will be rescued during a 72-hour ceasefire, it is understood.
Planners are looking at other options, including pulling some out by sea in a mission involving a Royal Navy Type 23 frigate, HMS Lancaster, and a Royal Fleet Auxiliary landing ship dock, Cardigan Bay.
Speaking on Tuesday evening Rishi Sunak said there would be "many more" flights out of Sudan tomorrow , with more than 1,000 people contacted and many making their way to the airfield.
The PM said the next 24 hours are "absolutely critical" in the evacuation of British nationals.
He added that he he could not "guarantee" the long-term safety of the route given the volatility of the ceasefire - but other options were being considered.
Asked about accusations that the Government is not helping citizens in the capital, the Prime Minister said: "I'm pleased that we were actually one of the first countries to safely evacuate our diplomats and our families.
"And it was right that we prioritised them because they were being targeted.
"The security situation on the ground in Sudan is complicated, it is volatile and we wanted to make sure we could put in place processes that are going to work for people, that are going to be safe and effective."
UK passport holders who make it aboard the aircraft will be flown from an airfield outside the capital Khartoum to the British airbase RAF Akrotiri in Cyprus, No10 confirmed on Tuesday.
The Sudanese airstrip has been secured by German troops who are aiding the evacuation operation.
It is understood British forces will take over the guarding of the airfield tonight.
The Prime Minister’s spokesman said: “We are continuing to operate evacuation flights for as long as possible, with consideration to the risk to both UK personnel and British nationals.
“I can’t speculate on how long that is going to be.”
Quizzed about the time window in which British nationals can flee Sudan, he admitted: “Given the volatile and dangerous situation, we cannot predict for how long those flights will be available, how long it will be safe to facilitate flights from that airstrip.”
Deputy Prime Minister Oliver Dowden also hosted a meeting of Whitehall’s Cobra emergencies committee.
Mr Sunak, who gave the green light for the rescue mission late last night, updated the Cabinet on the Sudan crisis at its weekly session this morning, including on the rescue of UK diplomats from the war-torn African nation early on Sunday.
The PM’s spokesman said: “He said government had been working at pace over the weekend both to bring about an end to violence and to evacuate diplomatic staff who were facing a specific threat to their safety.
“He paid tribute to those involved in the successful operation.
“He said that in parallel, the Foreign Office and Ministry of Defence have been working on ways to support British nationals who wish to leave Sudan and that he had now authorised a wider evacuation plan.”
When evacuees arrive at the British base on the Mediterranean island, the Government “will be facilitating their travel to the UK”, said the spokesman.
Africa minister Andrew Mitchell said there is going to be a "humanitarian catastrophe" in Sudan.
He told the Foreign Affairs Committee: "There is going to be a humanitarian catastrophe there with the position at the moment in terms of food and the humanitarian relief. Most of the humanitarian workers are leaving and they will not be able to come back until there is a ceasefire."
The minister added: "Five of them have already been murdered."
Asked if the Government should have seen this coming, Mr Mitchell said: "I think that is very difficult. My answer is no.
"This isn't an ideological battle, this is a battle between two generals for power. There is no ideology involved in this, it's raw power being fought over.
"One might have hoped that this would never happen given the appalling humanitarian jeopardy it has placed so many people in."
He added: "We would not have expected that these two generals would have slugged it out in this way on a totally non-ideological issue, let alone deploy heavy weapons in built-up areas."
The Mirror told today how 71 NHS doctors are among those stranded, prompting calls for them to be prioritised in a rescue operation.
Should the Sudanese airfield no longer be an option, plans have been drawn up to rescue British nationals by sea, Defence Secretary Ben Wallace told MPs.
Appearing before MPs on the Commons Defence Select Sub-Committee, he said the Red Sea city of Port Sudan would be an “alternative location”.
The Type 23 frigate HMS Lancaster has been ordered to sail to the port, while the Bay-class landing platform dock RFA ship Cardigan Bay “will be making its way at some stage” to Port Sudan.
“While it is 500 miles away from Khartoum across the desert, it is a more benign environment and it is an access into the Red Sea and commercial airports and places like Jeddah, and gives us space and time in order to process any people there,” he said.
“We already have people there, we’ve put some people in from the Royal Marines to work alongside to establish the safety of the area and any options.
“We are in a good position there, should we wish to increase support.”
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