Robin Cook last night launched a searing attack on the US and British governments for their prosecution of what he called a "bloody and unjust war".
It is the first time since the start of the conflict that a leading British political figure has called for hostilities to be ended with Saddam Hussein still in power.
Mr Cook's call for an immediate withdrawal from the war zone is a warning to Tony Blair of the immense political problems ahead if – as is now feared – the conflict drags on and the coalition forces are obliged to lay siege to Baghdad.
The former foreign secretary broke the silence he has maintained since his resignation speech in the Commons nearly a fortnight ago, which was greeted with an unprecedented standing ovation from fellow Labour MPs.
Mr Cook's intervention will raise new doubts about whether Mr Blair can survive in office if the war is not over quickly. His opponents on the far left of the party issued a new call yesterday for his removal.
Writing in the Sunday Mirror, Mr Cook said: "I have already had my fill of this bloody and unjust war. I want our troops home and I want them home before more of them are killed."
He attacked Mr Bush for "sitting pretty in the comfort of Camp David" while Allied forces risked death in an "unnecessary and badly planned" war. "It is easy to show you are resolute when you are not one of the guys in a sandstorm peering around for snipers," he wrote. "Nobody should start a war on the assumption that the enemy's army will co-operate. But that is exactly what President Bush has done.
"And now his Marines have reached the outskirts of Baghdad, he does not seem to know what to do next."
He was scathing about the new tactic outlined by the US Defence Secretary Donald Rumsfeld, which Mr Cook summarised as sitting down outside Baghdad "until Saddam surrenders".
He warned: "There is no more brutal form of warfare than a siege. People go hungry. The water and power to provide the sinews of a city snap. Children die.
"There will be a long-term legacy of hatred for the West if the Iraqi people continue to suffer from the effects of the war we started."
Mr Cook revealed the thinking of many of those who sent the coalition into war, confident of a quick victory. "Shortly before I resigned, a Cabinet colleague told me not to worry about the political fallout – the war would be finished long before polling day for the May local elections. I just hope those who expected a quick victory are proved right."
He commended the decision to bring back the bodies of slain troops for burial in Britain, but added: "I can't help asking myself if there was not a better way to show consideration for their families.
"A better way could have been not to start a war that was never necessary and is turning out to be badly planned." Mr Cook's call for an immediate end to the war was echoed by Doug Henderson, who worked with him in the Foreign Office as Minister for Europe.
He told BBC Radio 4 that the only alternative was an escalation of the conflict, dragging in Syria and possibly Iran. "I think a ceasefire and withdrawal is by far the better way forward," he added.
Downing Street played down Mr Cook's comments and insisted that the war would be fought to the finish.
A spokesman said: "Robin Cook has a well-known position on Iraq and it is not one that the Government shares.
"As the Prime Minister said in the press conference in Camp David, we will see the military campaign through until we achieve our objectives: that is, Saddam gone and Iraq's weapons of mass destruction disarmed."
Meanwhile, some of the Prime Minister's most hardened opponents were meeting in London yesterday to plan how they could "reclaim' control of the party.
Mark Seddon, editor of the left wing newspaper, Tribune, urged the 300 delegates to a Labour Against the War conference to set up a new organisation to "reclaim'" control of the Labour Party.
Later, delegates voted by more than two to one to campaign for a change in the party leadership..
Although Labour Against the War has relatively little support inside Parliament – mainly from hardened left wing MPs – what will worry Mr Blair is the links it has established with several large trade unions, including the GMB general union and the CWU postal workers union.