The UK's only Green MP Caroline Lucas has announced she is standing down at the next general election - likely to be in 2024.
The MP for Brighton Pavillion since 2010 and former Green leader said in a letter to her constituents that the "threats to our precious planet" had become "ever more urgent".
But she said her role in the Commons meant she had "struggled to spend the time I want on these accelerating crises".
"I have therefore decided not to stand again as your MP at the next election," she added in comments first reported The Argus newspaper in Brighton.
Ms Lucas, who has increased her majority at every election since entering Parliament, said it had been the "privilege of my life to serve this extraordinary constituency and community".
The Green MP said her focus on being "first and foremost a good constituency MP" meant she had "not been able to focus as much as I would like" on the climate emergency.
Ms Lucas was Green Party leader between 2008 and 2012 before returning for a second stint at the helm, this time co-leading the party with Jonathan Bartley for two years from September 2016.
The Green Party's co-leader Carla Denyer heralded Ms Lucas as a "force of nature" when paying tribute to her activism.
She said: "Caroline's impact on politics in this country cannot be overstated."
Joint leader Adrian Ramsay said the party, which made gains at the local elections in England last month, would be focused on electing new representatives to replace Ms Lucas, who remains the only Green candidate to have made it to the green benches.
"Having MPs who are genuinely dedicated to standing up for the climate and nature could not be more important than it is right now and that's why we are striving to get more Green MPs elected at the next general election so that we can build on Caroline's achievements," he said.
