Claire Short described military action against Iraq as "very unwise" yesterday, hinting that she might resign from the Cabinet if Britain backed strikes against Saddam Hussein.
Her comments, which echo concerns from Labour backbenchers, will intensify pressure on Tony Blair to draw back from supporting US President George Bush in any strike against President Saddam.
The Prime Minister already faces intense under pressure from within his own party, with Labour backbenchers making up the bulk of more than 100 MPs who have signed a Commons motion declaring their "deep unease" over military action.
The pressure intensified yesterday as the former Northern Ireland secretary Mo Mowlam, warned that Britain was drifting towards and "offensive, not defensive" war in Iraq.
Writing in the Sunday Mirror, she said: "Blair seems to be making it clear that he has more sympathy with the wishes of Washington and their reckless attitude to Iraq than he does for his own party and even members of his Cabinet."
It also emerged that David Blunkett, the Home Secretary, had warned the Government of the danger of civil disorder should strikes be launched on Iraq. Reports said Mr Blunkett had pointed to the possibility of increasing tensions in the Middle East spreading to Europe.
Ms Short told the BBC's On the Record programme: "The best thing is to get the UN inspectors back here, but there isn't crude military action that can deal with the problem of Saddam Hussein, and with the state of the Middle East and the terrible suffering of both the Israeli and Palestinian people. [With] the anger there is in the Arab world, to open up a military flank on Iraq would be very unwise."
Pressed on whether she might contemplate resignation, she said: "Yes, I am the same old Clare Short, and I'm proud to be a member of the Government, but I've got lots of bottom lines, but I don't expect the Government to breach them, but if they did I would ... That's what you should be like in politics I think."
Ms Short insisted the West "must not ignore" President Saddam's determination to develop weapons of mass destruction, but said there were more sophisticated responses than "instant mass bombing".
She said: "My view is very strongly that we should face up to how serious this is. I mean, chemical and biological weapons are almost more frightening than nuclear in that you don't need complicated machinery to deliver them.
"A little bottle of anthrax in a river in any country could kill lots and lots of people, so we can't ignore this.
"We need a much more sophisticated debate about what's the best way to deal with it."