Former Labour MP Alice Mahon has resigned from the party saying she has "lost faith with it".
Mrs Mahon, 71, was a member of Parliament for Halifax from 1987 until 2005, when she quit the Commons.
She considered resigning in 2005 but remained to see if Gordon Brown could make Labour "caring". Mrs Mahon added: "I couldn't have been more wrong".
A Labour Yorkshire and the Humber spokesperson said she had been "a good servant for the people of Halifax".
Mrs Mahon, who had "totally disapproved of everything Tony Blair was doing", told the BBC she "couldn't have stood another term under his leadership".
Alice Mahon: 'It's taken a long time to come to this decision'
But when he stood down she stayed in the party, hoping for a change of direction when Gordon Brown became leader.
"I hoped we might go back to being a caring and progressive party. In the event I couldn't have been more wrong."
Mrs Mahon said her decision to leave the Labour Party had a lot to do with its policies, claiming many promises in the 2005 election manifesto had been broken.
"For example we said that we would not privatise the Royal Mail, and we're going ahead and privatising part of it," she said.
"We said we would give people a referendum on the [EU] constitution. And yet the Lisbon Treaty has just been rushed through Parliament with no referendum and that's another promise we've broken."
"It is not a party I recognise - I have lost faith with it," she added. "I am very very sad, the Labour Party has been my life."
Mrs Mahon said it had taken "months of agonising to reach my decision."
She said the final push came after the recent scandal surrounding emails sent by Mr Brown's special adviser, Damian McBride, who sent e-mails proposing unfounded slurs about senior Tories.
Mrs Mahon is also angry at the government's Welfare Reform Bill saying it was "an assault on people with disabilities and the poor" and that it was remarkable it was being inflicted "in the middle of a recession".
I have reached the conclusion that there is not any avenue left in the structure of the Labour Party for people like me.
She expressed her frustration at the failure of the government to tackle the excesses of UK bankers.
"There is bewilderment that as a government we have not taken on the bankers, who are still doing as they please and taking large bonuses," she said.
She also lambasted the "energy fat cats" who were "hiking up prices".
"Government seem unwilling or unable to take them on, we expect the Labour Party to take people on in that situation."
"I have reached the conclusion that there is not any avenue left in the structure of the Labour party for people like me.
"Any threat from anybody marginally from the left and the party machine comes down on them like a ton of bricks."
Her resignation letter has been sent to the Halifax Constituency Labour Party.
A Labour Yorkshire and the Humber spokesperson said: "Alice Mahon has been a long standing member of the Labour Party and a good servant for the people of Halifax, it is a shame that she no longer feels she can continue as member of the party.
"However the Labour Party in Halifax and across the country will continue to make sure people get the real help they need in these difficult economic times."
During her time in the Commons she was one of Labour's most consistent backbench rebels, opposing the government on a range of issues from cuts in lone parent benefits to National Missile Defence.
She is a self-proclaimed socialist with a long record of peace activism and was one of the most vocal critics of the war in Iraq.
Mrs Mahon has also spoken on feminist issues, and against curbs on abortion.
In addition she is a strong supporter of gay rights and reform of the House of Lords.
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