A Former Labour MP has quit the party after half-a-century in the wake of the government's e-mail smears scandal.

Veteran left-winger Alice Mahon says she was “shocked and absolutely scandalised” by Downing Street official Damian McBride’s attempts to smear top Tories.

Mahon, 71, MP for Halifax between 1987 and 2005 accused the party of betraying its principles and was on course for election defeat.

Ms Mahon said it was a difficult decision to quit the party, but said: “I can no longer be a member of a party that at the leadership level has betrayed many of the values and principles that inspired me as a teenager to join.”

Ms Mahon, who opposed the Iraq war, said that she had been unhappy with the direction Tony Blair had taken the party and hoped Gordon Brown’s leadership would have improved matters, but added: “I couldn’t have been more wrong.”

In particular she attacked the smear campaign, cooked up between Brown’s spin doctor Damian McBride and Labour blogger Derek Draper, for personally targeting David Cameron and his wife in the wake of their son Ivan’s death.

“Like everyone else I think that most decent people in the party would be shocked and absolutely scandalised by the smears that were about to be launched on our behalf.

“I cannot imagine what kind of person would think it is a good idea to smear a couple who have just lost a loving son - I really can’t.”

She said there was a ‘desperate and dispirited’ feeling among many Labour party members at moment and when asked if she thought Labour would lose the next General Election, she added: “Unless there is a dramatic change in what we are proposing to the electorate then that could be the case.”

Her resignation comes as deputy leader Harriet Harman has called for renewed confidence in the party and Gordon Brown's leadership.

Speaking at today's gathering of the Labour LGA group she said: “I think now is a time for us to be confident and for us to be determined,”

“Every day it is clearer that we have the answers to the big questions and the big future challenges. So we should be confident, in our record, in our values, in our leadership and in our team.”

Ms Harman went on to say that the party was “fortunate to be able to look to the leadership” of Mr Brown.

She added: “He is demonstrating conviction leadership.”

Last week Gordon Brown was forced to make a belated apology to senior conservatives including David Cameron and George Osborne over the e-mails.

Among the unsubstantiated rumours being proposed by McBride as material a new pro-labour blog called Red-Flag were that Cameron could have an embarrassing sexual disease and Osborne’s wife was emotionally unstable.

McBride was forced to resign after the e-mails were leaked to right-wing blogger Paul Staines and subsequently published.

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