Ex-Cabinet minister Clare Short quit as a Labour Party MP today to campaign for a hung Parliament to replace Tony Blair's "arrogant, error-prone" Government.
Ms Short resigned the party whip and announced she would sit as an independent in the Commons until she steps down altogether after the next general election.
In her resignation letter - seen by the media before party chiefs, to their fury - she said she wanted to be free to air her concerns.
Her move comes shortly after she was slapped down by Chief Whip Jacqui Smith for urging voters to abandon Labour to force change.
Ms Smith, who stopped short of stripping her of the whip at the time, said it was "a shame" and attacked Ms Short for telling the media first.
"Many people remember her contributions to the Labour Party and Government and it is a shame that, following my reprimand to her for advocating the defeat of her Labour colleagues at the next election and several recent warnings about her lack of attendance, she has decided to resign the Labour Whip," the Chief Whip said.
"It is unfortunate she decided to announce this through a leak to the media rather than to her constituency Labour Party."
The chairman of Ms Short's Ladywood constituency, Malcolm Speak, said he was "not altogether surprised" at the decision.
But he pointed out that Ms Short had publicly supported "much of what the Government is doing".
The local party would "continue to support the work of the Labour Government even if Clare no longer feels she can", he added.
Even though the Chief Whip had not received the letter, Ms Short's name was immediately removed from the party's website.
In her letter, Ms Short wrote: "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."
She went on: "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."
She complained that Ms Smith's predecessor as chief whip Hilary Armstrong had "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".
She went on: "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."
She said she remained "proud of the history of the Labour Party and a convinced social democrat".
Mr Speake said: "We are not altogether surprised that Clare has resigned from the Labour whip.
"Clare's criticism of the Labour Government since leaving the Cabinet has been well known.
"It is only surprising inasmuch as Clare's support for much of what the Government is doing and has done is also well documented.
"In her 1997 election manifesto Clare said 'a strong dynamic economy is the essential foundation stone to get Britain moving again' - Labour has delivered that. The economy is stronger than ever.
"In 2001 and 2005 Clare recognised the achievements of the Government for her own constituents. In 2001 she said that the Labour Government had delivered extra funding for local schools and the health service.
"In 2005 Clare supported Labour's management of the economy and praised the 21% cut in jobless figures in Ladywood and the 30% drop in crime.
"Here in Ladywood Labour Party we will continue to support the work of the Labour Government even if Clare no longer she feels she can.
"It is a shame that Clare chose to announce this to the media rather than to the constituency party that has campaigned for her over many years."
Earlier this month, Ms Smith issued Ms Short with a formal reprimand over comments she made this summer.
Party chiefs appeared to decide stripping her of the whip could have inflamed the controversy and made her a "martyr".
In that broadside, she said she was "profoundly ashamed" of the Government and claimed Tony Blair's "craven" support for US policies had made the world a more dangerous place.
The future of British politics lay in a hung parliament because it would encourage electoral reform, she suggested, in direct defiance of the advice of Ms Smith.
She was stepping down, after 23 years as an MP, in order to "speak the truth and support the changes that are needed", she said at the time.
"I will pay whatever price because in the end you shouldn't be in politics if you can't speak your truth."