US President George Bush this morning gave Saddam Hussein 48 hours to leave Iraq with his sons or face the "full force and might" of Allied military force.

Starting the countdown to probable airstrikes early on Thursday or Friday, a grim-faced Mr Bush used a 15-minute TV address from the White House to tell Saddam that unless he complied, the US would launch war "at a time of our choosing".

Mr Bush was speaking hours before a Commons debate in which Tony Blair was due to ask MPs to authorise the use of "all means necessary" to disarm Iraq.

It follows the Prime Minister losing his Leader of the Commons - Robin Cook quit the Cabinet last night.

Mr Blair could face a second resignation today with International Development Secretary Clare Short - who has publicly threatened to quit - said to be "reflecting overnight" on her position.

Mr Blair is also facing probably the largest backbench rebellion of his premiership, with up to 120 MPs queuing to sign an anti-war motion tabled late last night.

Meanwhile, the US President said there was "no doubt" Saddam held weapons of mass destruction and it was necessary to disarm him now before he became able to use them in concert with terror groups to kill thousands of Americans.

"Instead of drifting along towards tragedy, we will set a course towards safety," said Mr Bush. "Before the day of horror can come, this danger will be removed."

Resolutions passed at the time of the 1991 Gulf War gave authority to disarm Saddam by force, claimed Mr Bush, adding: "The UN Security Council has not lived up to its responsibilities, so we will rise to ours."

He sent out a personal message to Iraqi troops not to obey orders to use chemical or biological weapons or set fire to oil wells, and urged them not to oppose Allied troops entering their country.

And he promised ordinary Iraqis that the US would help them rebuild their shattered nation after any conflict, telling them: "The tyrant will soon be gone. The day of your liberation is near."

The Baghdad regime was defiant yesterday, with foreign minister Naji Sabri saying that "any child" in Iraq knew that the demand for Saddam to go was a non-starter.

"The only option is the departure of the warmonger number one in the world, the failing President Bush who made his country a joke," he said.

United Nations Secretary-General Kofi Annan issued an order for weapons inspectors and UN humanitarian staff to leave Iraq. They are expected to begin pulling out today.

Mr Cook's decision to go followed the announcement by Britain's ambassador to the UN, Sir Jeremy Greenstock, that the Government had finally abandoned its increasingly futile efforts to secure a new UN resolution authorising war.

Sir Jeremy said that Britain, the US and Spain would not be putting their draft resolution to a vote.

After a Cabinet meeting yesterday, Deputy Prime Minister John Prescott blamed French "intransigence" for the diplomatic failure, saying that if the international community had remained united, Saddam could have been disarmed "without a shot being fired".

Mr Cook's resignation came as no surprise after he signalled his deepening reservations at the prospect of military action without a specific UN resolution at last Thursday's meeting of the Cabinet.

In his letter of resignation to Mr Blair, Mr Cook wrote: "I believe it is against Britain's interests to create a precedent for unilateral military action."

Later, Mr Cook was given an unprecedented standing ovation in the Commons as he said he would vote against the Government's plans to go to war.

Making a personal statement to explain his decision, he told MPs: "I cannot support a war without international agreement or domestic support.

"Neither the international community nor the British public are persuaded that there is an urgent and compelling reason for this action in Iraq."

Although he has looked an increasingly isolated figure in Government since he was removed as Foreign Secretary after the last General Election, he could prove a formidable rallying point for Labour critics of war.

Earlier, the Government attempted to shore up its position with the Attorney General Lord Goldsmith setting out the legal justification for military action.

There were reports of intense political arm-twisting in the corridors of Westminster, with some MPs complaining that ministers were warning potential rebels that they could bring down the Government.

Seven Cabinet ministers - including Mr Prescott, Defence Secretary Geoff Hoon and Welsh Secretary Peter Hain - were said to have been working the Commons tea-room, where MPs gather.

* Britain is swinging behind war with Iraq, a survey suggested last night.

Backing for military action has risen nine per cent to 38 per cent in the past month while opposition fell eight per cent to 44 per cent, according to an ICM poll for The Guardian.

Satisfaction with the Prime Minister's performance was up five per cent to 40 per cent while those unhappy with him fell four per cent to 51 per cent.