Protests are taking place in York and other big cities

He said Britain and the US were still working hard to get a second resolution authorising force against Saddam Hussein, but said previous resolutions had already authorised war.

On Saturday Prime Minister Tony Blair is to speak to ministers and telephone fellow leaders as part of a last-ditch attempt to resolve the crisis peacefully.

He will then join US President George Bush and Spanish prime minister Jose Maria Aznar in the mid-Atlantic Azores islands on Sunday.

It is something we just don't want to think about Soldier, on the prospect of nerve gas contamination UK troops mark 'final countdown'

Thousands of people have joined anti-war demonstrations in cities across Britain on Saturday, including York, London, Portsmouth, Leeds, Exeter and Newcastle.

Speaking on BBC Radio4's Today programme, Mr Straw stressed to go to war without the UN resolution would not be illegal.

"What we are doing is upholding the authority of the United Nations," he said.

"Any action we are involved in or in the future will be involved in will be fully consistent with our obligations under international law."

Legal 'belief'

He said he had been involved in detailed negotiations for the previous UN resolution 1441 and fellow signatories "knew what they were signing up for".

Its warning of "serious consequences" for Saddam if he did not disarm meant force - even if others including UN secretary general Kofi Annan thought only "all necessary means" meant force, he said.

It is in the British interest for us to support him (Tony Blair) if he does what is right for Britain Iain Duncan Smith, Tory leader IDS gives party support to Blair

Mr Straw says the UK and US are pushing for a new resolution for political rather than legal reasons.

He said: "There is a difference between whether you've got legal authority - which, as I say, we believe we have - and whether we thought the overall political support for any military action, if it's necessary, could be enhanced with... unanimity."

But a group of left-wing MPs has challenged the foreign secretary to prove that an attack on Iraq without UN backing would be legal.

Opposition

The Socialist Campaign Group of Labour MPs has written collectively to the speaker of the House of Commons saying an explanation is crucial before a vote is taken on whether or not to go to war.

Liberal Democrat leader Charles Kennedy has also questioned Mr Straw's certainty over the legality of military action.

Speaking ahead of the Lib Dem's weekend spring conference in Torquay, he told the BBC: "International lawyers are themselves divided over this.

"I have asked the prime minister in the course of the past few days to publish the legal evidence and advice he's received... which the government are reluctant or unwilling to do."

Mr Kennedy said the summit appeared to be a "final military council" before war.

Protesters carrying banners blasting Mr Blair marched through York on Saturday on their way to a rally at York Minister.

As well as Saturday's demonstrations there will also be an anti-war concert called One Big No, featuring Paul Weller, in the capital.

Britain's Iraqi community is also voicing its opposition to the French stance over possible conflict in the Middle East.

Supporters of the Iraqi Kurds are due to protest at France's vow to block a resolution permitting war. They cite years of oppression and torture carried out under Saddam Hussein's regime as justification for war.

Britannia Airways and Air 2000 both refused to comment on reports that injured British troops could be brought back to the UK on commercial planes.

The MoD is said to have contacted the two airlines. An MoD spokesman was not specific over which airlines were contacted and said the move was purely a contingency measure to boost air transport capacity.