Special forces are on standby to evacuate Brits trapped in Sudan as the country is ravaged by civil war.
Up to 120 SAS and a company of paratroopers are at the forefront of a plan to airlift embassy staff and UK citizens.
The US, France and China are also poised to get their people out as the conflict escalates.
More than 400 people have been killed and 3,500 injured, while 20,000 are said to have fled to neighbouring Chad, says the UN.
Capital Khartoum has been the centre of two weeks of fighting between the army and a paramilitary group.
They are led by two rival generals – who yesterday agreed to help foreigners evacuate by air.
There are about 50 staff at the UK embassy, hundreds of British nationals and thousands of civilians with dual nationality.
A Sudanese former minister told the BBC: “There are rotting bodies of our youth in the streets. We are out of water for six days.”
An evacuation plan would be similar to one carried out in Libya in 2012 when RAF Hercules aircraft, supported by the SAS, rescued hundreds from desert landing strips.
It is being led by Lieutenant General Charles Strickland, a former Royal Marine with combat experience in Afghanistan.
A source said: “The situation is extremely volatile. Going into Khartoum looks unrealistic. The best option may be evacuation from outside.”
The UK Government said it was preparing for “a number of contingencies” and “doing everything possible to support British nationals and diplomatic staff”.
