Caroline Lucas is to step down as the co-leader of the Green Party of England and Wales in September.
The party’s first and only MP – for Brighton Pavilion – has shared the role with Jonathan Bartley since 2016.
In an article for the Guardian , Ms Lucas said Greens “must always be an insurgent force for good, consistently asking the big questions that matter”.
She said a new leadership team “will step up to the challenge that our increasingly febrile times present”.
Ms Lucas, 57, said she would be “focusing even more” on her work in Parliament and in her constituency, rather than standing for re-election.
Green Party leaders are elected for two years and a fresh election is due to take place this summer, with nominations opening in June and a result announced in September.
She took Brighton Pavilion from Labour in the 2010 general election.
“I believe that Jonathan and I have shown the power of working together, since we became joint Green Party leaders in September 2016, and it’s now time for me to show the power of letting go,” she wrote.
Ms Lucas said the party’s structures had been updated in the last two years, which was one of the co-leaders’ priorities in 2016.
She added that the Green Party must consistently ask “the big questions that matter… and give the people of this country the bold alternative that is so desperately needed”.
In 2007, the Greens moved away from their longstanding tradition of having two principal speakers in favour of electing a leader every two years.
A former MEP, Ms Lucas won leadership elections in 2008 and 2010 but decided against standing in 2012, when she was succeeded by Natalie Bennett who served until 2016.
Mr Bartley, elected as a councillor in May, has yet to say whether he will stand again.
Paying tribute to Ms Lucas on Twitter, he said he was “so proud” of what the duo had achieved over the last two years.
— Jonathan Bartley (@jon_bartley) May 30, 2018
Other senior figures in the party, including MEP and finance spokeswoman Molly Scott Cato, have said they owe Ms Lucas a huge debt of gratitude.
I’m sure I speak for many, many others when I express my enormous gratitude to you for doing such a brilliant job – and continuing to make a powerful impact at Westminster https://t.co/Hd8O5nz7Fh
— Molly Scott Cato MEP (@MollyMEP) May 30, 2018
The party’s share of the vote dropped by 2% at the 2017 general election, but it won 39 council seats – an increase of eight – in May’s local elections in England.
Speaking at the time, Ms Lucas said: “The Green Party has taken a significant step forward with just a fraction of the resources of the bigger parties. We are now established as one of the four major English parties – and an electoral force right across the nation.”