Yemen risks becoming "a much more serious threat" to UK national security, Foreign Secretary William Hague warned amid fears it could descend into chaos.
Mr Hague accepted that the troubled Middle East country risked falling to pieces as its president went abroad for medical treatment to wounds sustained in a rebel rocket attack.
And he renewed his appeal to British nationals there to flee the violence while they still had the chance - warning they should not expect a Government-led emergency evacuation.
The prospect of a violent power grab mounted after president Ali Abdullah Saleh flew to Saudi Arabia for urgent medical attention after the attack on his compound in the capital Sanaa.
He was reported to have taken his family with him and Mr Hague said it remained unclear whether he intended to return to the increasingly violent confrontations with tribal leaders.
Its history as a breeding ground for al Qaida terror attacks means the UK and allies have been particularly concerned to prevent Yemen becoming a failed state.
But Saleh has repeatedly refused to sign up to a mediated agreement tabled by neighbours in the Gulf Co-operation Council - leading to escalating violence.
Asked about the latest developments, Mr Hague told BBC1's Andrew Marr Show he was "very worried".
Successive British Governments had worked hard to achieve stability there, he pointed out, urging the president to agree the deal.
"We have not succeeeded in that but we will continue working very hard on that. It could become a much more serious threat to our own security," he went on.
In a fresh warning to Britons still in the country - believed to number a few hundred - he said they should leave immediately while commercial flights were still available.
"They should not assume that in these circumstances, we can safely conduct or would try to conduct an evacuation," he said.