An extra RAF evacuation flight for British nationals still trapped in Sudan was due to depart on Monday.
Foreign NHS staff and their dependants with the right to live in Britain were also eligible for what is being billed as the last UK airlift from Sudan.
The UK has so far carried 2,122 people on 23 flights.
People needed to reach to the airport in coastal city Port Sudan, 500 miles from capital Khartoum where previous planes took off, by noon local time.
The Foreign Office would not confirm whether the plane had taken off. But flight tracking websites showed a Royal Air Force Hercules transport aircraft departed Port Sudan New International Airport at 18:43 local time (17:43 BST).
Another flight, an RAF Atlas transport plane, was due to leave at 20:25 local time (19:25 BST).
Foreign Office minister Andrew Mitchell warned on Saturday that the UK “can’t stay [in Sudan] forever” as the security situation continued to deteriorate.
Airstrikes and fighting have been reported over the weekend despite a ceasefire between rival army factions.
Tens of thousands of people have fled the country since fighting engulfed the country more than two weeks ago.
The capital city Khartoum has seen the heaviest fighting, with the Sudanese military and the Rapid Support Forces, a powerful paramilitary group, fighting for control of the country.
Sudan’s military said on Saturday it was launching a major new offensive against RSF positions in Khartoum.
The latest truce, which has not held, was due to end at midnight on Sunday. But the RSF said the ceasefire had been extended for another three days.
British security services had been scoping out Port Sudan as an alternative evacuation site since the beginning of the week, and have established a limited diplomatic presence there.
The British Ambassador to Sudan is leading the UK’s regional response from Addis Ababa in neighbouring Ethiopia.
The FCDO said the dependants of British nationals and NHS staff, who have leave to enter the UK, would also be allowed on the flight.
The UK government on Friday announced non-British NHS staff in Sudan could catch evacuation flights out of the country, in a U-turn on its previous policy.
Defence Secretary Ben Wallace said: “I am grateful to our armed forces who have ensured there was an alternative to Wadi Saeedna and who are currently supporting FCDO and Border Force staff to facilitate the rescue effort.”
A Royal Navy frigate – the HMS Lancaster – and Royal Air Force personnel are in Port Sudan, Mr Wallace added.
Another British vessel – the RFA Cardigan Bay – is on its way to Sudan and would also be able to provide humanitarian support, the BBC has been told.
The UK initially faced criticism for starting its evacuation after other countries, including European nations which rescued hundreds before the first British airlift took place.
A separate operation days earlier saw special forces troops evacuate UK diplomats from Khartoum after fighting broke out around the embassy.
But a Foreign Office spokesperson said the UK’s Sudan evacuation had grown to become “the largest of any Western country”.
The UN’s top humanitarian official, Martin Griffiths, says he is travelling to Sudan to co-ordinate the international aid effort, and would be looking to bring immediate relief to millions of people who have fled their homes.
Source : BBC