Anyone with British relatives or friends in strife-torn Yemen is being urged to contact them and plead with them to get out of the country while they still can.
The emotional warning issued by the Foreign Office yesterday is another sign that the credit that Yemen's long serving President has in Whitehall has run out. It was coupled with a veiled warning from a Foreign Minister, Alistair Burt, that, if his troops continued killing civilians as he clings to power, he could face trial in an international court.
The urgency of the Foreign Office advice suggests that Yemen is now classed alongside Libya and Somalia, where civil government collapsed 20 years ago, as one of the three most dangerous places in the world.
Mr Burt said yesterday: "Our clear message to British nationals still in Yemen is that they should leave immediately, while commercial flights are still operating.
"We cannot expect forewarning of any airport closures, and, if the situation deteriorates, it would be extremely difficult for the British government to assist its nationals in Yemen to reach safety. I cannot stress this too strongly. I ask those in the UK with friends, relatives and loved ones in Yemen to tell them to leave."
Mr Burt's statement went on to condemn the most recent reports of civilian deaths "in the strongest terms". He added: "The Yemeni government and security forces must exercise restraint and fulfil their responsibility to protect the Yemeni people, and the fundamental rights and freedoms they are entitled to. They must hold to account those responsible for excessive use of force. The reach of international justice is long, and the regime should note this.
"We and our partners in the Gulf region and elsewhere have repeatedly called on President Saleh to sign and implement the Gulf Co-operation Council agreement. He must do this, if Yemeni lives are to be saved, and if the country is to have a chance of orderly transition."
Since the breakdown of negotiations between the Yemeni opposition and the government of President Ali Abdullah Saleh on 22 May, the Foreign Office has pulled all but a skeleton staff out of the British embassy in Sanaa. It is thought that most roads leading out of the Yemeni capital are blocked.
The warning to get out of Yemen fast contrasts with the advice to Britons in Bahrain, where civil rights demonstrations by the majority Shia population have been violently suppressed by troops loyal to the Sunni-dominated government, backed by Saudi forces.
The advice to Britons there is to "maintain a high level of security and exercise caution, particularly in public places". Those who have no "essential" business in Bahrain are being advised to leave.
In Syria, where civil rights demonstrators have also been killed by government troops, the Foreign Office is saying that "British nationals who have no pressing need to remain should leave now by commercial means."
Until recently, President Saleh was seen by Britain as a force for stability in the Gulf and an ally against al-Qa'ida, which was using his country as a base for terrorists attacks abroad.
In January 2010, Gordon Brown hosted a London summit on Yemen after it emerged that the terrorist who attempted to blow up a plane over Detroit had met up with al-Qa'ida in Yemen. Politicians from all parties agreed then on the need to increase support for President Saleh's government to prevent the country falling into anarchy. The Department for International Development had already announced that aid to Yemen would be increased five-fold, to £50m a year, by 2011.
But President Saleh, who has ruled the northern part of Yemen since 1978, and the whole country since unification in 1990, forfeited his support in Whitehall when he failed to seal an agreement with the opposition, condemning his country to seemingly endless civil conflict.