'With so many opportunities to make a strong green voice heard outside the Green Party, I see no point in squandering my time, energy and spirit fighting endless redundant skirmishes behind the Green Party barricades,' she wrote in a resignation letter.
Her decision to withdraw her candidacy comes when the balloting for the next chair is already underway.
A year ago the membership voted for several reforms - known as Green 2000 - intended to streamline decision-making and give the party a clearer image and a more coherent voice during the general election. It was decided to have just two principal speakers - there had been 32 national speakers, an 11-strong executive and a chair.
Ms Parkin and others had been campaigning for such reforms for years, facing strong resistance from die-hard Greens who felt the soul and spirit of the party demanded that it be a highly devolved, grass roots organisation.
When the breakthrough for the reformers came last autumn a slate of Green 2000 candidates was elected to the key posts and the executive, including Ms Parkin.
But the changes reaped no electoral rewards. Although the Greens fielded more parliamentary candidates than ever before they got only 1.3 per cent of the votes on 9 April. They were firmly back in the electoral wilderness after their high point in 1989, when they received 15 per cent of the votes - more than two million - at the European parliamentary elections.
The party has grave financial problems and its image has not been helped by the strange developments in the life of one its best known figures, the former BBC sports presenter and ex-Coventry goalkeeper David Icke. In 1991 he announced that he had become the 'Son of Man' and 'an aspect of the Godhead'. He foretold that the earth was doomed to destruction in 1997 unless people underwent profound spiritual change which would allow the reign of God to begin with the new millennium.
The party also lost support as its mainstream rivals developed 'greener' agendas.
Since the general election Mallen Baker, a former co-chair of the party, has been campaigning with other activists for Ms Parkin's removal. Last night he hailed her announcement as a victory.
He said she was one of a small group of party leaders who had refused to work with activists, and had proved to be 'politically incompetent' and 'ineffectual'. 'Sara has shown herself to be completely unprepared to work with anyone who disagreed with her . . . her constant destructive behaviour has been doing the party real harm. Just because she is one of our best known faces and looks good on television does not mean she can be allowed to run the party badly,' Mr Mallen said. He is a candidate for the post of principal speaker in the current party elections.
Speaking from her home on the Scottish island of Isla, Ms Parkin said Mr Baker's choice of language showed what she was up against. 'I seem to spend all my time wrapped up in disputes with activists who are opposing the reforms and the new approach which the membership want.'
She said several other members of the executive were not standing for re-relection because of the infighting. Her letter withdrawing from the ballot read: 'I have been forced to the conclusion that the Green Party is a liability to green politics . . . There is a small but determined minority within the party able to completely negate the objective and spirit of those organisational changes.' Ms Parkin said she would remain a party member - she has belonged for 15 years. Aged 46 and with two sons, she said she now had many other things to do to promote the Green cause. Her lecture audiences include middle-ranking NATO officers.
Peter Barnett, the party's press officer said: 'We are obviously disappointed. We regard Sara as our greatest asset. We hope she will return as an active member.'