Demands for a no-fly zone over Libya have been backed by the Arab League in what the Foreign Office described as a "very significant" step.
Britain and France have been promoting the option of a no-fly zone as the international community considers its response to Colonel Muammar Gaddafi's reprisals against Libyan rebels.
Ministers have stressed the need for a proper legal basis and broad support, including in the Arab world, for such a move.
EU leaders stopped short of referring directly to the prospect of a no-fly zone, despite calls from Libyan rebels for one to be imposed, at the end of a meeting in Brussels on Friday. The US has appeared ambivalent on the issue, saying a no-fly zone may have limited impact.
But, gathering in Cairo, the 22-member Arab League backed the idea and called on the UN Security Council to put it into effect.
A Foreign Office spokesman said the Arab League had shown that Gaddafi's response to the uprising against him did not have support in the region.
"In brutally repressing a popular uprising by his own people, it is clear he is isolated and ignoring the will of the international community," the spokesman said. "No-fly zones are one option being considered as part of international contingency planning to respond quickly to events on the ground as they develop.
"This planning does not pre-judge any particular outcome. The Arab League call for a no-fly zone is very significant and provides important regional support to the option of creating a no-fly zone."
In a statement, the Arab League urged the UN to "shoulder its responsibility... to impose a no-fly zone over the movement of Libyan military planes and to create safe zones in the places vulnerable to airstrikes".
It rejected "all kinds of foreign intervention" in Libya but warned that "not taking the necessary action to end the crisis will lead to intervention in Libya's foreign affairs".
