Britain has joined growing calls for the possible imposition of no-fly zone over Libya in an effort to prevent further attacks by Colonel Muammar Gaddafi on his own people.

David Cameron told the Commons he had ordered the Chief of Defence Staff, Sir David Richards, to begin work with key UK allies on plans for a military no-fly zone.

The Prime Minister said the international community could not tolerate Col Gaddafi's "illegitimate" regime using military force on the civilian population and he warned that they needed to be ready to act if the repression worsened.

"We do not in any way rule out the use of military assets. We must not tolerate this regime using military force against its own people," he told MPs.

"In that context I have asked the Ministry of Defence and the Chief of the Defence Staff to work with our allies on plans for a military no-fly zone."

He added: "Of course we must comply with international law, but my argument is that we need to do the preparation and the planning now because no one can be sure what Col Gaddafi will do to his own people.

"If he starts taking that sort of action we might need to have a no-fly zone in place very quickly."

US Secretary of State Hillary Clinton, attending the United Nations Human Rights Council in Geneva, said that America was also actively looking at the possibility of a no-fly zone.

"The no-fly zone is an option we are actively considering. I discussed it with allies and partners," she said. "All options are on the table. That of course includes a no-fly zone."

The Libyan deputy permanent representative to the United Nations, Ibrahim al-Dabashi - one of a number of diplomats to desert the regime - said it was "very important" to have a no-fly zone to "prevent the dictator using helicopters to terrorise people in the streets".