The first RAF flight carrying British nationals has left Sudan, with two more trips to take place overnight, Downing Street has said.
UK citizens were being processed for evacuation at an airfield near Khartoum on Tuesday after an RAF mission was launched during a "volatile" ceasefire brokered between the warring factions.
The Prime Minister's official spokesman later confirmed that the first departure had taken place from the Wadi Saeedna site, headed for RAF Akrotiri in Cyprus.
"The first flight has left and you can expect that there will be at least two more flights overnight tonight but that is subject to change," the spokesman said.
The situation is "fast-moving" but the flight that has already left is expected to arrive in Cyprus later, Downing Street said.
People will be moving "fairly rapidly" from RAF Akrotiri to the UK, it added.
Defence Secretary Ben Wallace said around 120 British troops are supporting the operation at the Wadi Saeedna airfield, which is currently being secured by the German military.
UK forces have the "capacity" to take over should allied troops leave, he said.
British passport holders are being urged to make their way to the airfield where priority will be given to the most vulnerable, with more than 2,000 citizens having registered in Sudan with the Foreign Office.
Mr Wallace told the Commons Defence Committee that Royal Marines are also scoping out a possible seaborne evacuation from the more "benign environment" of Port Sudan, some 500 miles from the capital.
Royal Navy vessels HMS Lancaster and RFA Cardigan Bay are in the region and are on standby to assist with the evacuation if required.
UK's military support
Around 1,400 military personnel are said to be involved in the evacuation effort, which was launched after a 72-hour ceasefire was agreed by Sudan's warring factions.
RAF flights are coming from RAF Akrotiri and the evacuation plan involves similar aircraft to those used to rescue diplomats and their families from Sudan on Sunday – A400M and C-130 Hercules transport planes.
Prime Minister Rishi Sunak authorised the operation late on Monday night after facing criticism for failing to airlift more than the British diplomats and their families over the weekend.
Foreign Secretary James Cleverly said earlier Sudan remains in a "dangerous, volatile and unpredictable" state despite the ceasefire.
Mr Cleverly said UK nationals would have to make the risky journey to the airbase near Khartoum without a military escort and warned it is "impossible" to know how long the pause in the fierce fighting will last.
He warned the pause – agreed by the two rival generals in Sudan – is fragile after speaking directly or through intermediaries with faction leaders as he called for them to allow British nationals to be evacuated.
Mr Cleverly also defended the Government from suggestions it should have carried out evacuations of citizens sooner, as European allies had succeeded in doing.
"The circumstances for each individual nation are different. There are considerably more British nationals in Sudan than other countries have got," he said.
An RAF C-130 Hercules that left Sudan earlier and returned to Cyprus was understood to have been carrying an advance team.
The C-130 is the RAF's primary tactical transport aircraft, described by the Air Force as "highly flexible" and the "backbone" of UK operational tactical mobility missions since entering service in 1999.
Mr Sunak this morning described the evacuation as "large scale" as he paid tribute to the military, diplomats and Border Force staff conducing the "complex operation".
He added Britain will work to "end the bloodshed" in Sudan, which was triggered by two rival generals engaging in a power struggle, and support a democratic government.
Later addressing his Cabinet, the Prime Minister said there had been a "specific threat" to the safety of diplomats before their evacuation on Sunday.
Families with children or elderly relatives, or individuals with medical conditions, will be prioritised for the flights.
Only British passport holders and immediate family members with existing UK entry clearance are being told they are eligible.
Nationals have been warned that all travel within Sudan is "conducted at your own risk".
Shadow foreign secretary David Lammy said he was "extremely relieved to hear of the short but desperately needed ceasefire", but "the Government must now work with great speed to ensure as many British nationals still in Sudan can be rescued as quickly and safely as possible".
More than 420 people, including at least 273 civilians, have been killed since fighting began on 15 April, and a further 3,700 have been wounded.
Although 2,000 UK citizens have registered with the British Embassy for evacuation, it is thought the total number of British nationals in Sudan could be up to 4,000.