The prime minister has insisted that Britain and the US would win UN backing for a war with Iraq.

Tony Blair argued that the UN will agree a resolution backing force should evidence emerge that the Iraqi regime is flouting existing weapons resolutions.

But he warned his international counterparts that they could not use a second resolution to let Saddam Hussein off the hook.

"We must disarm Iraq of weapons of mass destruction, we must uphold the authority of the United Nations, we must show rogue states and terrorist organisations that when we say we intend to deal with the issue of weapons of mass destruction we mean it," said Blair on Monday.

Briefing journalists inside Number 10, the prime minister defended his tough line on Iraq despite growing backbench unease at the direction of Downing Street thinking.

He said voters should wait a couple of weeks for evidence of Saddam's weapons arsenal to emerge.

Blair said the government knew the public was concerned about the possibility of war - but insisted that Saddam Hussein must be disarmed by whatever means necessary.

He said that the Iraqi regime had provided an "inadequate and probably false" account of its weapons arsenal.

"If there is a breach... I have no doubt that the right thing to do is to disarm Saddam Hussein by force," said Blair.

He insisted that "action would follow" any credible evidence of a breach of UN resolutions.

"Of course no-one wants conflict. Everyone would want this to be resolved peacefully. It is not conflict that is inevitable it is disarmament that is inevitable," said the prime minister.

Blair said that failing to act "would send a message to the outside world that would be absolutely disastrous to the security of that world".

But he insisted that he would "never send British troops into war unless I thought it was necessary".

And he dismissed suggestions of a cabinet rift - insisting that all ministers hoped the UN route would deliver disarmament.

The prime minister added that it was "naive and misguided" to suggest that the UK was simply responding to US demands.

"If George Bush and America were not raising this issue I would be urging them to do so," said Blair.

His comments came as a new poll found that 58 per cent of the public do not believe Saddam Hussein poses enough of a threat to justify going to war.

They also followed Clare Short's latest intervention in the Iraq debate.

Speaking to ITV's Jonathan Dimbleby programme on Sunday, the international development secretary said she "absolutely" wanted to see a second resolution before any military action begins.

"I absolutely want a second resolution, but I don't want to confine what we say to a second resolution, because you can imagine a scenario where a resolution is passed that authorises any action," she said.

"If the authority of the UN is not honoured - as it was not before, when the weapons inspectors were thrown out - then there is some action we have to take to reinforce the authority of the UN."

And pressed on whether Britain should rule out participating in any unilateral military action by America, she responded: "That is the logic of the position."

Her views have the support of public opinion, according to the latest survey.

A YouGov poll for ITV News found just 13 per cent would back British involvement in military action against Iraq without United Nations backing.

But with full security council authorisation 53 per cent would support the participation of UK troops in military action.

Surveys at the weekend also indicated that war without a second UN security council resolution would see Labour members leaving the party.

The Conservatives argued that public doubts about military action reflected the uncertainty caused by splits in the government.

Speaking on Sunday, Iain Duncan Smith also accused the prime minister of "wobbling".

"I'm not in favour of simply following the Americans. I think we have a requirement to make the case for British involvement on British interests, protecting British citizens. That is the key," he told BBC1's Breakfast with Frost.

"My concern is that Labour and the prime minister have not made that case. They have failed to advance the case, because they are so split and divided over this issue.

"That worries me, because it means the British people are still waiting to hear what the case is for British involvement.

"We are now beginning to see the prime minister wobbling on this and not showing the lead. This is the man who is promising a lead but not giving it."