Outspoken Birmingham politician Clare Short has resigned as a Labour MP, claiming she is sick of being gagged.
Ms Short will continue to represent her Ladywood constituency until the next General Election, but will sit as an independent.
Her move follows a series of clashes with Tony Blair over Iraq and public service reforms.
But the final straw for Ms Short was a written reprimand from the Chief Whip, Worcestershire MP Jacqui Smith.
Ms Smith (Redditch) disciplined Ms Short after she called for a hung Parliament which, in practice, would mean Labour losing seats. Ms Short insisted she could only be free to express her opinions by resigning from the Parliamentary Labour Party.
She would sit in the Commons as "an independent Labour MP" and continue to be a "convinced social democrat", she said.
Officially, Ms Short remains an ordinary member of the Labour Party, despite quitting as a Labour MP, but she is now likely to be expelled.
She said in her resignation letter: "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."
Ms Short accused Ms Smith of trying to silence her through "a string of rebukes".
The letter added: "In the circumstances, I think the best way to ensure that I can put forward my views for my remaining time in Parliament is as an independent Labour MP."
Speaking after her resignation became public, Ms Short said: "There are so many things that the Government has done wrong that are so unprincipled so un-Labour.
"I think the Government has lost its way.
"I said a long time ago the act of going to Iraq made him an unfit Prime Minister. I hoped the Labour party would tell him to stand down."
Last night Ms Smith said: "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 resignation letter was posted to Ms Smith on Thursday using the House of Commons internal mail system, but the Labour leadership does not appear to have been aware of it until yesterday.
Last night, colleagues expressed anger at her decision. Khalid Mahmood (Lab Perry Bar) said: "I'm disappointed. Clare has taken a huge amount from the Labour Party.
"Now she has let down the activists who delivered leaflets for her in the rain over 20 years and she has let down her constituents who stuck by her."
The chairman of Ladywood constituency Labour Party, Malcolm Speak, said he was "not altogether surprised".
Ms Short has been at loggerheads with the Prime Minister since quitting her post as International Development Secretary in 2003, after the Iraq War.
She has previously called on him to resign, and accused him of misleading the British public in what she called "an honourable deception" over Iraq.
She has already announced she will not stand again in Ladywood and the Party is to look for a new candidate.
Despite the many controversies, Labour has stopped short of expelling her so far, after senior figures warned against making her a "martyr".
Ms Short said in her letter: "I want to advocate the possibility of changing the voting system and the Chief Whip keeps threatening me.
"So I feel the only thing to do if I want to speak the truth is to resign the Whip."
She added: "I just want to be able to say what I want in the House of Commons."
Ms Short accused the Chief Whip of sending her a "threatening letter", and revealed she had spent a week agonising over her decision to resign.
She attacked Mr Blair's foreign and security policies, saying: "The whole approach to civil liberties and the war on terror is making the problem worse.
"We appear not to have learnt any of the lessons from Northern Ireland."