Britain's recognition of Libya's rebel National Transitional Council was "irresponsible and illegal," Libyan Deputy Foreign Minister Khaled Kaaim said on Wednesday.
"We consider this irresponsible, illegal and in violation of British and international laws," he said.
Kaaim said the regime of Libyan strongman Muammar Gaddafi "will take necessary actions," and would argue against London's decision before tribunals in Britain and other international courts.
He also poured scorn over British Foreign Secretary William Hague saying he was "acting in an unacceptable and not objective way."
Earlier on Wednesday Britain recognised thel NTC as the country's legitimate government after dramatically expelling all remaining diplomats loyal to Gaddafi from the London embassy.
Hague said he had invited the NTC to take over the embassy and appoint an official envoy in a major boost for the movement fighting Gaddafi's regime.
"The prime minister and I have decided that the United Kingdom recognises and will deal with the National Transitional Council as the sole governmental authority in Libya," Hague told a news conference in London.
"We are inviting the National Transitional Council to appoint a new Libyan diplomatic envoy to take over the Libyan embassy in London."
He added: "In line with that decision we summmoned the Libyan charge d'affaires to the Foreign Office today and informed him that he and the other regime diplomats from the Gaddafi regime must leave the UK."
"We no longer recognise them as the representatives of the Libyan government."
NTC recognition
Britain's move comes nearly two weeks after the Libya contact group, a group of major Western and regional powers, recognised the the NTC as Libya's legitimate government.
Britain is one of the lead nations in a NATO-led Western alliance that has been carrying out an aerial campaign against Gaddafi's regime since March, when the UN approved action to protect civilians.
On Monday Hague reiterated Britain's demands for Gaddafi to step down but said the Libyan leader may be allowed to remain in the North African country in an apparent shift in London's position.
Britain expelled the Libyan ambassador in May following attacks on the British embassy in Tripoli, and has also already kicked out several Libyan diplomats including the country's military attache.
NTC's 'COMPETENCE' RECOGNISED
It has no diplomatic representation in Tripoli but a Foreign Office special representative is based in the eastern rebel stronghold of Benghazi where the NTC is based.
"We will deal with the NTC on the same basis as other governments around the world," Hague said.
"We are dealing with them as if they are the state of Libya," he said.
"This decision reflects the National Transitional Council's increasing legitimacy, competence and successs in reaching out to Libyans across the country.
"Through its actions the National Transitional Council has shown its commitment to a more open and democratic Libya, something that it is working to achieve through an inclusive political process.
"This is in stark contrast to Gaddafi whose brutality against the Libyan people has stripped him of all legitimacy."
Hague said the decision could also help with the unfreezing of some assets which have been frozen by the international community to stop Gaddafi profiting from them.
Hague was meanwhile scathing about the "pretty worthless" medical advice that allowed the freeing of convicted Lockerbie bomber Abdelbaset Ali Mohmet al-Megrahi, who appeared at a rally in support of Gaddafi on Tuesday.
Megrahi, 59, who has terminal cancer, was released from a Scottish jail on compassionate grounds in August 2009. He is the only man convicted over the 1988 bombing of Pan Am flight 103 which killed 270 people, mostly US nationals.
