Clare Short angrily resigned yesterday as a Labour MP, condemning Tony Blair for "half truths and deceits" over the war on Iraq and accusing his Government of being "arrogant" and "error-prone".
The former Cabinet minister furiously attacked the Chief Whip Jacqui Smith for bringing about the "breaking point" that forced her to quit the Labour whip in the Commons after 23 years.
She also accused the Labour leadership of dirty tricks by leaking her letter of resignation to the press, saying it had been posted privately to the Chief Whip on Thursday.
News of her resignation broke as Ms Short was visiting a local hospital in her Birmingham Ladywood constituency.
But Ms Smith hit back, accusing Ms Short of leaking her decision to the media. She said: "It is unfortunate she decided to announce this through a leak to the media rather than to her constituency Labour Party."
As the furious recriminations continued, Ms Short spoke of the pain of her decision to resign the Labour whip. Speaking to The Independent, Ms Short said she wanted to stay on as a Labour member but would sit as an independent MP until the election, when she will retire from Westminster.
However, there is likely to be a move by her local Birmingham Ladywood constituency party to throw her out.
Ms Short said she had agonised over her decision, but decided to resign because of the threats she had received from the Labour Chief Whip.
She was reprimanded for writing in The Independent that she hoped there would be a hung Parliament at the next election that would lead to reform of the voting system and more democratic checks on the power of Downing Street.
In her letter of resignation, she defiantly repeated her call, saying: "Given that the next election might well produce a hung parliament, I want to be free to argue that this creates a valuable opportunity to reform our voting system so that the House of Commons more accurately reflects public opinion and we have a parliament more able to hold the Government to account..."
Ministers brushed aside her departure last night. One said: "Clare had become pretty semi-detached." But she remains a popular figure who will now be free to hound Mr Blair until he resigns from office next year.
Ms Short had grown increasingly disenchanted with Labour after resigning over the war on Iraq. She said she had become "ashamed" of the Government under Mr Blair, whom she accused of deception.
Her resignation as a Labour MP was announced without her authority as she visited a hospital in Birmingham. She was furious that the Chief Whip claimed that it had been Ms Short who informed the press.
Ms Short accused Mr Blair of lying to her to keep her in the Government when she had threatened to resign from the Cabinet before the war on Iraq. "There is a famous allegation against me that I didn't resign at the time of the war on Iraq. That was because Tony Blair had entered into a negotiation with me to try to get me to stay. I said to him, if you can't stop the war, then what is crucial is we move forward on the road map for the Middle East, and get the Palestinian state established, and we absolutely internationalise the reconstruction. It would be led by the UN and backed by the international community. He promised those things and it wasn't true. He just said that to fool me, but I believed he was sincere." She added: "I still think, even though the rush to war was long, if those things had been done, the Middle East would be an entirely different place and Iraq would be a totally different place than it is today.
"That is the biggest error he has made. It is such a serious problem for the world, it strengthened al-Qa'ida, it strengthened the willingness of people to contemplate the use of violence, it is breaking up the international system at a time when we need more international cooperation than ever to deal with environmental threats and global warming.
"But then there is the issue of top up fees, all the erosion of civil liberties, cash for peerages - more and more errors have come piling on to depart more for social democratic values that have made it more difficult to support the Government."
She was sceptical that Gordon Brown - a former close ally - will change Labour when he takes over from Mr Blair. "It's still very likely to be Gordon and I hope he changes the policy on Iraq. It would be wonderful if he does so. It's always important to get change but I think so much has gone on it's going to be very difficult to bring things around to what could have happened.
"What I want is a chance to speak honestly without a constant stream of complaints from the Chief Whip that I can't advocate a change in the electoral system."
Former defence minister Peter Kilfoyle, a staunch opponent of the Iraq War, said: "I am sad she has chosen to resign. I would ask what she hopes to achieve by resigning. No matter how strongly you feel about issues, you should stay in the party and fight your cause."
Labour peer Lord Foulkes, who was her deputy at the Department for International Development, commented: "I always thought this was inevitable. It is a bit like the final act of a modern Greek tragedy."
A Conservative Party spokesman said: "The Labour Party is divided and paralysed, and it is no surprise that Clare Short has decided to jump ship."
Resignation letter to Jacqui Smith, Chief Whip
"I am sorry it has come to this but after a lifetime of service to the Labour Party and 23 years in the House of Commons, I think I am entitled to discuss what has gone wrong with the Government and our political system in my remaining years as an MP.
"It is my view that our political system is in trouble and that the exaggerated majorities in the House of Commons have led to an abject parliament and a concentration of power in Number 10 that has produced arrogant, error-prone government. Given that the next election might well produce a hung parliament, I want to be free to argue that this creates a valuable opportunity to reform our voting system so that the House of Commons more accurately reflects public opinion and we have a parliament more able to hold the Government to account and to ensure that policy is well considered.
"As you know, I am critical of many other aspects of Government policy. The previous Chief Whip tried to use her authority to stop me discussing the fact that the Prime Minister engaged in a series of half-truths and deceits to get us to war in Iraq. You focus on my views on electoral reform. The consequence is a string of rebukes, usually through the media. In the circumstances, I think the best way to ensure that I can put forward my views for my remaining time in Parliament is for me to resign the Whip. I will therefore sit in the House of Commons as an independent Labour MP.
"I remain proud of the history of the Labour Party and a convinced Social Democrat."