Around 4,000 UK passport-holders are still stranded in Sudan amid heavy fighting.
Priority will be given to people with disabilities, elderly people and families with children when RAF planes land at an airfield near Khartoum.
There are concerns over the viability of a 72-hour truce that began on Monday evening.
It is estimated that around 1,400 soldiers are involved.
Prime Minister Rishi sunak said that RAF aircraft had been involved in the large-scale evacuation and described it as a complex operation.
Foreign secretary says trapped Britons in Sudan will be contacted and told to stay away from the airfield until called.
The Foreign Office stated that “the situation is volatile and we may not be able to evacuate at any time.”
Around 4,000 UK citizens are believed to be trapped inside the country in east Africa as rival military groups battle for control.
Since the 15th April, hundreds of people have been killed. The evacuation follows days of intense pressure to devise a plan for the British.
Sky’s Alistair Bunkall said that many more evacuation flights are likely to be followed. He saw a flight depart from RAF Akrotiri, Cyprus, just before 7am.
Bunkall, the Foreign Office spokesperson, said that they are also considering other “points” of departure – perhaps by sea.
Some UK citizens managed to escape via evacuation flights operated by another country.
Germany, Italy and Spain are just a few of the countries that have rescued hundreds of passengers from dozens of different countries.
The Foreign Office stated that only British passport holders would be able to get a place on UK planes.