UK forces are preparing to help enforce a no-fly zone over Libya after the UN backed "all necessary measures", short of an invasion, to protect civilians.
Downing Street has cautioned against earlier suggestions that British planes could be in action "within hours" and declined to put a timetable on it.
The UN resolution rules out a foreign occupation force in any part of Libya.
The cabinet met earlier and Prime Minister David Cameron is now making a statement to Parliament.
MPs will debate the issue later following Mr Cameron's statement, and there is set to be a Commons vote on Monday, government sources have indicated.
Labour have said they back the no-fly zone - meaning the government would be likely to carry the Commons vote.
The UN resolution imposes a "ban on all flights in Libyan airspace", with aid flights the only exception.
It authorises member states to "take all necessary measures" to "protect civilians and civilian populated areas under threat of attack", including in the rebel stronghold of Benghazi, short of an putting an "occupation force" on the ground.
It also calls for an immediate ceasefire, an end to the violence, measures to make it more difficult for foreign mercenaries to get into Libya and a tightening of sanctions.
UK Foreign Secretary William Hague said it was a "positive response to the call by the Arab League" for measures to protect Libyan civilians and was the culmination of "a great deal of hard work in the last few days" by France, the UK, Lebanon and the US.
"It is necessary to take these measures to avoid greater bloodshed," Mr Hague said.
Ten UN Security Council members backed the resolution while five abstained - nine votes were needed for it to pass.
France, the UK, Lebanon, the US, South Africa, Bosnia-Hercegovina, Colombia, Portugal, Nigeria and Gabon all voted in favour, while China, Russia, Brazil, India and Germany abstained.
Libyan leader Col Muammar Gaddafi's forces have recently retaken several towns seized by rebels in an uprising.
Rebel forces in Benghazi reacted with joy to the passing of the resolution but Said Gaddafi, the Libyan leader's son, said the resolution was "unfair" as the regime had "proved to everybody that there have been no air strikes against civilians".
In response, Libya has also closed its airspace to all traffic, according to EU air transport authorities.
Loyalist forces are bearing down on Benghazi, home to a million people. Forces loyal to Col Gaddafi have reportedly launched their first air attacks on the town, targeting the airport at Benina.
Col Gaddafi had earlier warned the rebels there that his troops were coming and to expect "no mercy".
Senior UN sources had said British and French warplanes could be in the air within hours of the UN vote to carry out initial air raids on Libyan positions, possibly with logistical support from Arab allies.
But No 10 sources have declined to put any timetable on possible British military engagement - or whether action could begin this weekend. The BBC's Defence Correspondent Caroline Wyatt said the UK and its allies had to decide what their initial targets would be and who would conduct the missions.
The resolution would permit air strikes on Libyan ground troops or allow attacks on Libyan war ships if they were attacking civilians.
A former head of the UK's armed forces said the "endgame" was to remove the Gaddafi regime.
"You can't attack him, so the point is that his military capability is what is sustaining him in power at the moment and he is trying to roll the opposition forces back, so they become the focus of the allied and coalition efforts," Lord Dannatt told the BBC.
A draft UN resolution tabled on Tuesday proposed a ban on all flights in Libya, authorised member states to enforce it and called on them to participate in it.
But on Thursday the US said the UN should go further and a new strongly-worded draft resolution was put forward calling for "all necessary measures short of an occupation force" to protect civilians under threat of attack.
Following the UN vote Mr Cameron had a half-hour phone call with US President Barack Obama and a White House spokesman said the leaders would "co-ordinate closely on the next steps" in conjunction with other allies.
The unrest in Libya started after the toppling of the long-time leaders of neighbouring Tunisia and Egypt earlier this year.
Libyan protesters started to demand that Col Gaddafi step down after 42 years of autocratic rule and quickly seized much of eastern Libya.