International Development Secretary Clare Short is staying in her cabinet job despite saying she remains "very critical" of the way the Iraq crisis has been handled.
Ms Short has previously threatened to quit if war began without a new United Nations mandate, but leaving now would be "copping out" when Tony Blair had no other option.
She said she was so close to quitting that she had written her resignation statement on Monday.
Ms Short's decision is a boost for Tony Blair, who suffered a second and third ministerial resignation just hours before he asks MPs to back war with Iraq.
Home Office Minister John Denham and Health Minister Lord Hunt of Kings Heath both quit the government, following in the wake of cabinet minister Robin Cook, who objected to military action without a new UN resolution.
Ms Short told BBC News that the decision had been the hardest of her career.
She said: "I know that I will be vilified ... It is easier to go, I would be more popular if I went. But the truth is I would be copping out.
"If I think Tony (Blair) has no option then it is cheap of me to resign as though he could do something different now."
Ms Short, who this month called Mr Blair's policy "reckless", said the crisis had been badly handled but she had to look at the circumstances as they were now.
She explained: "Iraq is a mess. Saddam Hussein is defying the UN, the people are suffering. We have to go on.
"Nineteen million people need food aid. We have got to make sure they are cared for."
Impact of war
Ms Short cited the decision of America, under British pressure, to publish the roadmap to the creation of a Palestinian state by 2005 as a critical factor in her decision.
That commitment to justice in the Middle East was crucial, she argued.
She had also seen two detailed presentations by military officials about the possible impact of war on the Iraqi people.
"Obviously war is a risk and never ever should be undertaken lightly," she continued.
"The targeting is enormously careful to go for the things that will help the military and not the people."
Much of the military action would involve stabilising the country, keeping people fed, she argued.
"It is highly likely that the military action will be over in a matter of weeks, that the Iraqi forces will crumble very quickly because this is a very nasty regime and its people do not like it."
There were risks, however, including worries around the use of chemical or biological weapons.
It emerged that Ms Short was urged by UK aid agencies to stay in the cabinet.
On behalf of the British Overseas Aid Group (BOAG), Julian Filochowski, director of the Catholic aid agency, Cafod, spoke to Ms Short on Monday to press her to remain in her post "if she felt she could", said a Cafod spokeswoman.
Mr Filochowski has a close relationship with Ms Short and spoke to the international development secretary after consulting with other BOAG directors.
The group is made up of the five leading UK aid agencies - Cafod, Oxfam, Christian Aid, Action Aid and Save the Children.
Mr Filochowski told Ms Short she was needed to "create humanitarian space in the forthcoming conflict," said the spokeswoman.
Mr Blair's official spokesman indicated that Mr Blair was glad Clare Short had chosen to remain in the cabinet.
He said: "The prime minister has always made it clear that he values Clare Short as a cabinet minister particularly committed to her department's responsibilities.
"Those skills are particularly valuable given where we are."
The spokesman was asked whether Mr Blair had confidence in Ms Short.
He said: "He has full confidence in her as a cabinet minister with particular departmental responsibilities."
The spokesman said Ms Short had expressed concerns about two issues in particular, the first being the future of the Middle East peace process and the second the reconstruction of a post-war Iraq.
He added that those concerns had now been met.