Foreign Secretary William Hague has called on Muammar Gaddafi to recognise his 42-year rule over Libya was over and stand down his forces.
Mr Hague insisted the Libyan people had delivered a "decisive rejection" of the dictator and dismissed his vow to fight on as "delusional".
"I think it is time now for Colonel Gaddafi to stop issuing delusional statements," he said.
Gaddafi's grip on power was finally broken on Tuesday night when rebels stormed his heavily-fortified Bab al-Aziziya compound in Tripoli.
Jubilant rebels raised the new Libyan flag over buildings and looted personal possessions from the 69-year-old's luxurious home - including his famous colonel's hat.
But in a poor-quality telephone address on local television, the 69-year-old claimed he fled the base as a "tactical" move. He said he had gone on a discreet tour of the Libyan capital and did not feel that the city was in danger.
Gaddafi told residents they must "cleanse" Tripoli of the rebels and free it from the "devils" who have overrun it. The beleaguered tyrant vowed victory or death in his fight to remain in power.
Mr Hague said he did not know how long the "death throes" of the regime would take. But, after chairing a meeting of the National Security Council in London, he went on: "There is a clear, fundamental decisive rejection of the regime by the people of Libya ..."
Asked about the predicament of the Britons in the Rixos, Foreign Secretary William Hague said he was monitoring the situation "hour by hour".
He said: "We are also doing what we can to help through talking to the National Transitional Council, although they are not yet in control of that area, and to any others who may be able to help."
