Glenys Kinnock, wife of the former Labour leader, has warned Tony Blair against taking any military action in Iraq without the sanction of the United Nations.
Her warning is a sign of the uphill struggle facing Mr Blair as he tries to convince the Labour Party that Saddam Hussein must be divested of his weapons of mass destruction, with or without the backing of the UN.
Mrs Kinnock, a Labour MEP and a popular figure with conference delegates, told The Independent on Sunday: "We need to have an international mandate, and the only way to do that is through the UN weapons inspectors."
If the Iraqis refused to give unrestricted access to the inspectors, she added: "It should go back to the UN... The decision should not be taken by any member of the Security Council, either alone or in pairs."
Mrs Kinnock, whose husband played a major role in advancing Tony Blair's political career, claimed her remarks were aimed principally at US government hardliners, who appear to have scant respect for the UN. She urged Mr Blair to use his influence in Washington to persuade the US not to launch a unilateral strike.
Mr Blair has privately told members of the Cabinet that he is reluctant to launch military strikes against Saddam Hussein with the US alone. In public he has avoided any appearance of disagreement with President Bush, and has warned the UN must not be an "excuse" for inaction.
Mrs Kinnock said: "I don't think, in these difficult circumstances, that we can work on that presumption. I don't think it should be seen as an excuse, or a rubber stamp... Involvement of the UN is an international mandate. It's the only institution we have that we can go to in these circumstances."
Mrs Kinnock's statement means that the Labour Party's three most popular women politicians are all, to varying extents, critics of the possible war. The former minister Mo Mowlam sent a message of support to marchers taking part in yesterday's anti-war demonstrations, saying: "Go for it. Keep up the fight. It is crucial for the future of our country, our self-respect and our democracy."
And a week ago Clare Short, the International Development Secretary, warned against killing innocent Iraqis.
Labour's longest-serving MP, Tam Dalyell, yesterday described the "slide to war" with Iraq as the most dangerous situation since the Cuban missile crisis 40 years ago.
The party leadership is anxious not to allow Iraq to dominate this week's conference, but has been forced to allow a debate on it.
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