Liberal Democrat leader Charles Kennedy has added to growing pressure on Prime Minister Tony Blair to oppose a US-led invasion of Iraq.

There is no doubt that Saddam Hussein now has weapons of mass destruction

Dick Cheney, US vice-president

Mr Kennedy said he wanted to see evidence that Saddam Hussein was stockpiling weapons of mass destruction - and was willing to use them - before supporting another war.

Meanwhile, a survey of constituency party chairman suggests the Labour Party is in open revolt over the prospect of military action.

Only a handful of the 70 party chairmen surveyed by The Times newspaper said they would back military intervention.

'Not there yet'

The findings will send a fresh reminder to Mr Blair of the scale of opposition within his own party to military action.

While Mr Kennedy's intervention undermines the prospect of a cross-party coalition on Iraq, which Mr Blair will need if the UK is to commit forces to an American-led invasion.

If he (Saddam) had not been stupid enough to invade Kuwait in 1990 he would have had nuclear weapons by the middle of the 1990s

Lord Hannay, former ambassador to UN

Mr Kennedy told BBC Radio 4's Today programme: "If we can have clear evidence that there are weapons of mass destruction which are being amassed, with a willingness to use them on the part of Iraq, the international community will self-evidently have to act.

"But we are not there yet."

The Liberal Democrat leader called for renewed international pressure on Saddam to re-admit weapons inspectors.

He also called for evidence of a link between Iraq and the 11 September terrorist attacks on the US.

Nuclear threat

However, Britain's ambassador to the United Nations during the Gulf War, Lord Hannay, said doing nothing about Saddam was not an option.

George Bush has pushed for Iraq "regime change"

"I don't think we can simply sit back, cross our fingers and hope for the best that Saddam is not going to develop these weapons, which he was very close to getting in 1990.

"If he had not been stupid enough to invade Kuwait in 1990 he would have had nuclear weapons by the middle of the 1990s.

"And there is nothing any of us would have known about it."

Lord Hannay said there would be no legal obstacle to invading Iraq, as Saddam was in breach of "multiple obligations" he had accepted at the end of the Gulf War in 1991.

But past experience showed that the US would be unwise to act without as wide a consensus as possible.

He said the re-admission of weapons inspectors with "unfettered access" should be put to Saddam "very forcefully".

'Recipe for chaos'

Earlier, former cabinet minister Peter Mandelson, hinted the UK would seek UN authorisation before joining any US attack on Iraq.

Mr Straw wants to set Saddam a deadline

Mr Mandelson, who remains a close confidante of the prime minister, said: "As the judge and the jury in this case, it is the United Nations which should be the main pillar in a world based on law and international cooperation."

The former Northern Ireland Secretary, who is visiting Indonesia, said there was a case for international intervention against Iraq.

But he said a unilateral attack by the US would be a "recipe for chaos" in the Middle East.

His words come amid signs of a growing rift between the UK and US over Iraq.

Cheney warning

In a speech on Thursday, US Vice President Dick Cheney dismissed suggestions that weapons inspectors could be used to contain Saddam.

His words were in marked contrast to the line coming from London, where Foreign Secretary Jack Straw said he was considering setting Iraq a deadline for the re-admission of inspectors.

Mr Cheney told veterans of the Korean war: "There is no doubt that Saddam Hussein now has weapons of mass destruction.

"There is no doubt that he is amassing them to use them against our friends, against our allies and against us," he said.

"And there is no doubt that his aggressive regional ambitions will lead him into future confrontations with his neighbours."

Meanwhile, French President Jacques Chirac has become the latest major Western ally to speak out against unilateral American military strikes against Iraq, arguing that it was a matter for the UN to decide.