Britain's leading politicians have united behind the most significant decision to use the UK military in an international intervention since the 2003 Iraq invasion.
RAF Tornado and Typhoon jets are being deployed to airbases from where they will be able to help the international coalition implementing a no-fly zone over Libya.
British air-to-air refuelling aircraft and surveillance aircraft will also be deployed to forward airbases from where they will be in a position to take direct action against Libya, David Cameron told the Commons.
But the Libyan foreign minister said Muammar Gaddafi's government was prepared to implement an immediate ceasefire in the wake of last night's UN security council resolution.
The prime minister told MPs the decision to implement a no-fly zone was "extremely right". Labour leader Ed Miliband said he applauded the efforts of those who had brought about the no-fly zone, "including... the prime minister and the British government".
It follows the passing of a UN security council resolution authorising the use of "all necessary means" short of invasion to prevent Libyan leader Colonel Gaddafi using his jets against civilians.
"The world has watched as he [Col Gaddafi] has brutally crushed his own people," Mr Cameron told MPs.
He said there was a "demonstrable need" for the military action because Col Gaddafi was "preparing for a violent assault" on Benghazi, the one-million strong city regarded as the stronghold of rebel forces.
"In this country we know what Colonel Gaddafi is capable of," Mr Cameron continued.
"We should not forget his support for the biggest terrorist atrocity on British soil. We simply cannot have a situation where a failed pariah state festers on Europe's southern border."
He added: "Gaddafi has publicly promised every home will be searched and there will be no mercy and no pity shown. That is the demonstrable need."
The Commons will hold a debate on the decision to attempt to stop Col Gaddafi's forces entering Benghazi on Monday.
Mr Miliband responded to his statement by warning that any decision to commit British forces was "grave and serious".
But he added "it would be quite wrong for us to stand by, given what is going on in Libya, and do nothing".
A handful of Labour backbenchers raised concerns about the move, however. Left-winger Jeremy Corbyn said of Mr Cameron: "I hope he's thought this whole thing through because we may well be involved in a civil war in Libya for some time to come."
The prime minister responded by suggesting that "just because you can't do the right thing everywhere" did not mean not doing it where action could be taken.
Mr Corbyn's concerns were not isolated. Natascha Engel feared what would happen if "a complex and dangerous situation is simply made worse", while David Winnick warned that "we could be dragged into a third war in nine years".
Liberal Democrat MP Tom Brake said he wanted to know what would happen if the no-fly zone resulted in a stalemate on the ground between rebel and loyalist forces in Libya.
"There is a danger of stalemate," Mr Cameron conceded. "At that point there could be a role for the African Union to try and bring this situation to a close. But as we stand today Col Gaddafi has not ceased his attacks."
Libyan rebel forces, who had been calling for the international community to aid their cause, were seen celebrating the news of the security council resolution.
But the situation on the ground was thrown into doubt by the Libyan foreign minister's announcement of a ceasefire.
"Libya has decided an immediate ceasefire and the stoppage of all military operations," the Gaddafi regime spokesperson said, before calling for "dialogue" to take place.
"It's very strange and unreasonable where the security council allows the use of military power and that there are signs that this might indeed take place," the foreign minister added.
"It's a violation of the national sovereignty of Libya."
It remains unclear whether rebel forces will be able to resist the pro-Gaddafi military's steady advance towards the rebel stronghold of Benghazi, if fighting does continue.
Initial air raids on the city have targeted its airport and, this morning, reports emerged that four people had been killed and 70 wounded after an attack on Misrata.
Heavy bombing was reported on Ajdabiya, the final town on the Benghazi road where fighting has been at its heaviest in recent days.
UN security council resolution passed
The diplomatic breakthrough legitimising military action came yesterday evening when the UN security council voted 10-0 in favour of the resolution drafted by Britain, France and Lebanon.
"Gaddafi's regime has ignored this council's demand that it stop the violence against the Libyan people," Britain's ambassador to the UN, Sir Mark Lyall Grant, told the security council before last night's crucial vote.
"It is now preparing for a violent assault on a city of one million people that has a history dating back 2,500 years. It has begun air strikes in anticipation of what we expect to be a brutal attack using air, land and sea forces. Gaddafi has publicly promised no mercy and no pity."
The US had been slow to indicate its support for a no-fly zone, but was persuaded after the Arab League backed decisive action over the weekend.
Russia and China, traditionally suspicious of interfering in other countries' internal affairs, abstained rather than defy the wishes of those in the region.
"This resolution is an expression of resolve," foreign secretary William Hague said.
"We have said all along that Gaddafi must go, that the Libyan people must be able to have a more representative government and determine their own future. And it is necessary to take these measures to avoid greater bloodshed, to try to stop what is happening in terms of the attacks on civilians and on the people of Libya."
The opposition had earlier made clear it would not challenge Mr Cameron on his decision to press hard for military action against Col Gaddafi.
"I pay tribute to the work that Britain and other nations have undertaken to ensure that the security council have reached this important agreement," shadow foreign secretary Douglas Alexander said.
"The responsibility for this crisis rests squarely with the Gaddafi regime, and by this resolution the United Nations has now placed a responsibility on its members to act to protect the Libyan people."