In a letter sent to her Brighton Pavilion constituents, Ms Lucas said it had been the "privilege of my life to serve this extraordinary constituency and community".

But she said the "threats to our precious planet" had become "ever more urgent" and her role in parliament as the Green Party's only MP meant she had "struggled to spend the time I want on these accelerating crises".

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"I have therefore decided not to stand again as your MP at the next election," she said.

Ms Lucas, 62, made history when she was elected as the MP for Brighton Pavilion at the 2010 general election, becoming the first and only Green Party candidate to be elected to the House of Commons.

Her share of the vote has increased at every election since, with constituents returning her to parliament with a majority of almost 20,000 in 2019.

Colleagues from across the House of Commons paid tribute to her activism, with Labour's Chris Bryant calling her a "beacon in a dark time" and Lid Dem MP Layla Moran saying her departure is a "loss to parliament for sure".

In her letter, Ms Lucas 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 climate concerns.

She said: "I've done everything possible to help wherever I can and always worked to ensure that people feel heard, that their concerns matter, and that they are not alone.

"But the intensity of these constituency commitments, together with the particular responsibilities of being my party's sole MP, mean that, ironically, I've not been able to focus as much as I would like on the existential challenges that drive me - the nature and climate emergencies."

In her 13 years in parliament, she said her achievements include putting issues such as universal basic income and a legal right to access nature on the political agenda, campaigning for the new Natural History GCSE and securing the first debate on drug law reform.

She described herself as a "different kind of politician" - pointing to when she was arrested, charged and acquitted over a peaceful protest at Cuadrilla's exploratory fracking site in Balcombe, West Sussex, in 2013.

She added: "The truth is, as these threats to our precious planet become ever more urgent, I have struggled to spend the time I want on these accelerating crises."

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.

Sky News understands she is not stepping back from public life and will continue campaigning.

The Green Party's current leadership heralded her as a "force of nature" and said they would be "striving to get more Green MPs elected at the next general election so that we can build on Caroline's achievements".

Co-leader Carla Denyer said: "Caroline's impact on politics in this country cannot be overstated: she truly is a force of nature and has been an extraordinary servant of the people of Brighton Pavilion as well as the Green Party. We are so proud of her achievements."

Adrian Ramsay added: "Very few politicians can claim to have changed the course of the national debate in the way Caroline has. She has brought so much to us as a party, shown real integrity in her work and added a crucial dimension to our democracy in this country."

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Ms Lucas joins a growing band of senior elected politicians who have declared that they will step down at the next election, which is expected to be contested next year.

More than 50 MPs have announced an end to their Commons careers, including former SNP Westminster leader Ian Blackford, Conservative former deputy prime minister Dominic Raab and former deputy Labour Party leader Margaret Beckett.