RAF planes will fly into an airfield outside Khartoum and priority will be given to families with children, the elderly and people with medical conditions.

It appears to be race against time as there are fears over whether a 72-hour ceasefire, which began late on Monday, will hold.

Evacuation effort begins for stranded Britons - Sudan latest

Britons will also have to reach the airfield themselves - potentially encountering lingering fighting - as no escorts are being provided.

Foreign Secretary James Cleverly said it was "impossible for us to predict how long this opportunity will last".

Around 1,400 military personnel are believed to be involved in the UK operation.

Prime Minister Rishi Sunak tweeted that RAF aircraft were involved in the "large-scale evacuation" and called it a "complex operation".

Britons trapped in Sudan are being contacted directly and are being told not to go to the airfield unless they are called.

"This is an active conflict, the ceasefire has been announced but we know there have been pockets of violence even within previous ceasefires," said Mr Cleverly.

"So this does remain dangerous, this does remain difficult.

"We are providing what assistance we can and we are operating as quickly as we can."

He also said the situation was "fundamentally different" to the chaotic evacuation of thousands of people from Kabul when the Taliban seized control of Afghanistan in 2021.

About 4,000 UK passport holders are thought to be trapped in Sudan as rival military factions battle for control.

Hundreds of people have died since the fighting started on 15 April and the evacuation comes after days of pressure for a plan to get Britons out.

Food and fuel have soared in price, electricity and internet are cut off in much of the country and the clashes have left governments scrambling to get their citizens and diplomats out.

Mr Cleverly said contact had been made with leaders of the two factions "calling on them to allow British nationals, dual nationals and minors to be evacuated".

Sky's Alistair Bunkall - in Cyprus - saw a flight take off from RAF Akrotiri just before 7am and said many more are likely to follow.

The Foreign Office said it was also looking at other potential "points of exit" - possibly by sea, according to Bunkall.

Read more:
Which countries have evacuated their citizens from Sudan?
What's happening in Sudan?

Some UK citizens have manged to escape on evacuation flights operated by other countries.

Germany, Italy, Spain and France are among those that already rescued hundreds of people from dozens of countries on their own flights.

However, the Foreign Office said only British passport holders would be able to get a seat on the UK planes.

Britain's diplomats and their families were evacuated over the weekend in a precarious mission by elite troops that took place under the cover of darkness.

Some senior Foreign Office officials will be at the airstrip to coordinate the evacuations.

The violence in Sudan comes after rival generals fell out over a deal to incorporate the Rapid Support Forces (RSF) group into the military.

The army and RSF mounted a coup together in 2021 after long-time ruler Omar al Bashir was overthrown in a popular uprising two years earlier.

However, their relationship broke down during negotiations to integrate and form a civilian government.