Green Party leader Caroline Lucas says she will not be seeking re-election when her second term ends in September.
She announced on Sunday that she will continue as MP for Brighton Pavilion, and hopes to "dedicate even more of my work to the political front line".
She was elected as the party's first solo leader for England and Wales in 2008, after it switched from a system of having two principal speakers.
She became the party's first MP in the 2010 general election.
'Disastrous economic policies'
She tweeted that she was "standing down as Leader to open doors to new Green Party Leader".
"The reason that I've decided not to re-stand... is because I want to give other people the opportunity to get well known, to have some profile in the party, hopefully to use that to get themselves elected as well," she explained further to BBC Radio 4's Westminster Hour.
In her statement, she praised party gains in the past few years, including its first Westminster seat, first local council and its share of the vote in this month's London mayoral election and council elections.
"I'm hugely honoured to have served as the first leader of the Green Party and I'm proud that during the four years of my term, we've moved Green politics forward to a higher level, with the party by far the most influential it has ever been," she said.
"I look forward to continuing to do all I can in my very demanding role as the MP for Brighton Pavilion, representing my constituents and defending them against the coalition government's disastrous economic policies and its refusal to accept its environmental and social responsibilities."
Before she became leader she was the MEP for South East England.