Clare Short has been under pressure to stand up and be counted over Iraq - and she has finally done so.
Her devastating attack on Tony Blair flies in the face of Downing Street claims that ministers are "rock solid" behind the prime minister on this issue.
Few people really believed that anyway. But the outspoken assault by Ms Short and the resignation and threats of resignation by ministerial aides suggests the possible revolt is even bigger than previously feared.
The question it all raises is whether Ms Short is simply the tip of an iceberg that could sink Tony Blair's premiership.
It is absolutely the case that there are other cabinet ministers deeply troubled over the possibility of war against Saddam Hussein.
Robin Cook is persistently whispered to be the top of the list - but there are others.
And it seems highly likely that, if Mr Blair insists on going to war without a second UN resolution, they will feel compelled to speak out and possibly even follow Ms Short's example.
Future careers
For her part, the international development secretary has a record of resigning on matters of principle and the prime minister might just about be able to brush aside her protest.
He will almost certainly not sack her - yet. That would only succeed in creating a martyr and give the anti-war rebels a powerful new figurehead.
But she may not have done her long-term ministerial career much good.
Equally, the PPS's who have threatened to resign may be safe at the moment, but they have put their future careers firmly on the line.
Even if they do not quit now, as Margaret Beckett's aide Andy Reed has done, they know their futures are bleak.
Tony Blair could face cabinet resignations if he goes to war without UN backing
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If the prime minister emerges from the war victorious - with or without a second resolution - they will be in his little black book under the heading "eliminate".
Ms Short, if she has not already quit, will also be pencilled in for future retaliation.
Because, whatever Downing Street says, the prime minister will be furious that she has spoken out at such a sensitive time.
He is currently desperately making concessions in an attempt to win that vital second resolution sanctioning action against Saddam.
Serious revolt
The resignations and threats of resignations make that task far harder and seriously undermine his standing in the rest of the world.
These developments are all the more damaging because they add to the feverish atmosphere already gripping Westminster over the looming war.
They suggest the Labour rebellion, far from fading, is actually escalating.
It further raises expectations that, should the prime minister go it alone with President Bush, he may face a number of resignations from cabinet ministers.
That would be a far more serious revolt which would represent a major challenge to his authority.
It would split both the government and the Labour party wide open, with incalculable results.
If [Blair] goes to war without the second resolution, and it goes wrong, his leadership will be under threat
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What probably worries the prime minister most over these latest threats, however, is that it suggests MPs are seriously looking to life beyond his leadership.
It is no exaggeration to say that if he goes to war without the second resolution, and it goes wrong - with no weapons of mass destruction found or with huge civilian casualties for example - his leadership would be under threat.
Banking on resolution
A challenge would be almost inevitable and resignation could not be ruled out.
Mr Blair is clearly banking on winning a second resolution.
The prime minister's plan B if he fails to win the resolution, is that he still goes to war in the face of all the protests - but is proved right over weapons of mass destruction
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Under those circumstances, the majority of the rebels would fall in line and back him.
The country would also be expected to be on-side once war was underway and British troops were laying their lives on the line.
But that looks far from certain, although Downing Street continues to express its "confidence" that it will win the day at the UN.
Mr Blair's plan B if he fails to win the resolution, is that he still goes to war in the face of all the protests - but is proved right over weapons of mass destruction.
Odds still good
He is absolutely certain these weapons exist. If he goes into Iraq without UN backing but quickly uncovers stockpiles of such weapons, he will probably turn opinion behind him.
So the odds still look pretty good for Tony Blair - even though he knows he is taking by far the biggest risk of his political life.
What happens once the war is over is another question. There will be a legacy of bitterness and inevitable recriminations.
Even if Mr Blair attracts a post-war "Thatcher factor" - as happened after the Falklands conflict in 1982 - there will still be a large section of his party who will never again offer him unqualified support.