He told MPs a second United Nations resolution "would be preferable" before war - but did not rule out backing unilateral American military action.

Labour's Denis Skinner told Parliament during Prime Minister's Questions that a new Gulf War was about "America getting its hands on Iraqi oil".

He accused President Bush of "finishing the job his father failed to complete 12 years ago".

When Mr Blair tried to dismiss the left-winger's "conspiracy theory" he was heckled by some of his own MPs who shouted: "It's not a conspiracy theory".

Tory leader Iain Duncan Smith taunted Mr Blair over Cabinet divisions on Iraq saying ministers were split over the prospect of war.

He said: "You and your Cabinet must be clear and united in order to send the right message to Saddam Hussein, to British troops and to the British people."

Earlier Mr Blair was grilled by sceptical Labour MPs during a private meeting on Iraq.

He told a gathering of the Parliamentary Labour Party at Westminster that Saddam Hussein's regime had to be disarmed by any means possible - including force.

But his words did little to sway anxious Labour backbenchers. MP Glenda Jackson said she was not convinced by the argument for war.

"I have not been satisfied by what the Government's been saying on this issue all along," she said. "If no weapons of mass destruction are disclosed, what is the reason for going to war?"

Former defence minister Peter Kilfoyle added: "It was a typical PLP meeting and he got a good round of applause. But I'm never convinced by arguments on Iraq."

One MP said the meeting was split 50-50 over military action against Iraq.

Former culture secretary Chris Smith warned of "major disquiet" among MPs if war kicked off without a fresh UN resolution.

He said: "If America decides to jump the gun and take military action before the UN has either got the evidence or has taken a resolution and decided that this is what the world should do, if America does that, then I very much hope that Britain would side with the UN rather than the US.

"Because if the Americans decide to go ahead of a proper UN decision, then not only is there the potential for enormous humanitarian consequences, but also that would be so destabilising across the whole of the Middle East and it would really present major global problems.

"And I hope that in those circumstances that Britain would have the courage to say to the US, we don't think now is the time to do this."

Meanwhile General Wesley Clark, former Nato Supreme Allied Commander, said he thought America would attack in "mid to late February".

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