Natalie Bennett has announced she will stand down as leader of the Green Party in England and Wales this August, after four years at the helm of the party.
Ms Bennett said she would not stand for re-election when her second two-year term in office expires this summer.
The Australian former journalist said she intended to remain "fully engaged" in the Green Party after stepping down.
Green peer Baroness Jones and London mayoral candidate Sian Berry have said they do not want the job.
Ms Bennett said she had been "proud to lead a party through a period of phenomenal expansion and increased impact".
An ex-journalist who worked for the Telegraph, the Independent, the Times and the Guardian, she became party leader in September 2012.
She succeeded Caroline Lucas, the Green Party's MP for Brighton Pavilion.
Earlier this month, she defended her leadership after the Greens lost four councillors in local elections in England, and won a 2.5% share of vote in the Welsh Assembly elections, behind UKIP.
'Increased public understanding'
In a statement, Ms Bennett said: "With the support of our passionate members and supporters we have been able to achieve much in a relatively short period.
"There's greatly increased public understanding that when you want the honest, caring, committed view - one that isn't guided by the views of the latest focus group or fear of a tabloid backlash but by fundamental principles and values."
Ms Bennett took part in televised leaders' debates ahead of last year's general election.
Last year, she had to apologise to members following a radio interview in which she blamed "brain fade" for her inability to explain a series of party policies.
Speaking to the BBC following her decision to stand down as leader, she criticised the "soundbite" culture in politics and the media.
"I'm not smooth," she told Radio 4's World At One.
"Sometimes I get stuck for words. But what I think is, we also need to think about what kind of politics we want. Some of our political reporting is very much aggressive attacks on quite minor points, refusing to engage in the broader issues and that really doesn't do the public, the voters, any kind of favours."
Ms Bennett said she did not know who would run for Green Party leader.
Richard Mallender, chairman of the Green Party Executive, thanked Ms Bennett for "her outstanding leadership over the past four years."
Baroness Jones said it was a "really tough job" and she was "disappointed" Ms Bennett was stepping down.
"What we need now is some new people to come forward," she said. "There is incredible talent in the party who are not well known because they are not elected."
Nominations for Ms Bennett's successor will open on June 1 and close on June 30.
A campaign period will run from 1 July until 24 July, at which point a one-month balloting period will begin with the result to be announced at the party's autumn conference in Brighton in early September.