David Cameron has urged the European Council of Ministers to consider implementing a no-fly zone over Libya.
The prime minister joined forces with French president Nicolas Sarkozy, drafting a joint letter to put pressure on the Council's president.
The two European leaders said Colonel Muammar Gaddafi had lost legitimacy and demanded planning for "all possible contingencies".
Col Gaddafi's position appears to have strengthened in the north African country, as his military forces make inroads into rebel-held territory.
Yesterday, France recognised the rebel's Interim National Council in Libya. The decision was criticised in some quarters for bestowing legitimacy on a group about which little is known.
In Britain, foreign secretary William Hague continues to engage in dialogue with rebel leaders.