Clare Short has told BBC News she will resign from the government if Britain goes to war against Iraq without
United Nations backing.
British troops are nearly ready for action
The international development secretary told BBC Radio 4's Westminster Hour she could not "stay and defend the indefensible".
"If there is not UN authority for military action or the reconstruction of the country, I will not uphold a
breach of international law or this undermining of the UN," she told the programme.
"I will resign from the government," said Ms Short, who described Tony Blair's actions as "reckless".
Downing Street expressed surprise at Ms Short's comments, with a spokesman insisting she had not expressed such views before to the prime minister.
Home Office Minister Beverley Hughes said she was surprised by Ms Short's comments, which she argued should have been discussed with cabinet colleagues instead of on the airwaves.
Ms Hughes told BBC Radio 4's Today programme: "At a time of critical importance, when everybody, including the government, wants to get a second resolution, it is surprising to have a cabinet minister expressing her views on the radio, not to the prime minister and her colleagues round the cabinet table."
Britain, the US and Spain have put forward a draft resolution to the UN which sets a deadline of 17 March for Baghdad to disarm.
Downing Street insists that it is confident the Security Council will back the draft resolution in a vote, which is now likely to take place later in the week.
The UK and US are already planning concessions including agreement to table a list of specific demands on disarmament that Saddam Hussein must meet if he is to avoid a war.
It is also possible that the 17 March deadline for compliance by Baghdad could be extended.
Mr Blair spent Sunday at his Chequers official country residence, engaged in an intensive round of telephone diplomacy over the deadline idea.
But Ms Short, who resigned over Labour's support for the last Gulf War, said she was surprised at Tony Blair's "extraordinarily reckless" stance.
"The whole atmosphere of the current situation is deeply reckless - reckless for the world, reckless for the undermining of the UN in this disorderly world, reckless with our government, reckless with his own future, position and place in history."
Ms Short said she had raised her objections in frequent detailed discussions with both the prime minister and foreign secretary Jack Straw
"People like me are being told, 'Yes, all this is under consideration'," she said
AIDES 'THREATENING TO RESIGN'
Cambridge MP Anne Campbell, PPS to Trade and Industry Secretary Patricia Hewitt
Michael Jabez Foster, PPS to Lord Goldsmith, the attorney general
Great Yarmouth MP Tony Wright, PPS to Ruth Kelly, the Treasury minister.
"And then the spin the next day is, 'We are ready for war'."
It was time to put her "cards on the table" and reveal her intentions, Ms Short added.
"I feel the need now, because it is 10 minutes to midnight, to say out loud what I think Britain should do with its influence - because our failure to use our influence properly is so dangerous for the world," Ms Short said.
"We are undermining the UN," she added.
"Allowing the world to be so bitterly divided - the division in Europe, the sense of anger and injustice in the Middle East - is very, very dangerous," Ms Short told Westminster Hour.
"It is a recruiting sergeant for terrorism."
BBC political correspondent John Pienaar said Ms Short's threat had come as a surprise to Downing Street.
"She isn't just laying down this ultimatum, she is accusing Number 10 of reverting to old habits of spin, saying one thing in private and another thing in public."
Earlier on Sunday Loughborough MP Andy Reed announced he was quitting as parliamentary aide to environment secretary Margaret Beckett.
Three other parliamentary private secretaries - MPs who work as assistants to ministers - have publicly indicated they also would step down if action was taken without a new UN resolution.