Caroline Lucas is standing down as leader of the Green Party.
Lucas - who was elected as the party's first MP in 2010 - said she would step down in September at the end of her second, two-year term as leader.
"We're lucky to have a wealth of capability and experience in our party. Now feels like the right time to step aside, to allow more of that ability to come forward and help the party to grow," she said in a statement on the party's website.
"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 she would continue as MP for Brighton Pavilion "putting the Green case for change in Parliament".
Originally an activist with the Campaign for Nuclear Disarmament, Lucas joined the Greens in 1986.
In 1993 she achieved her first electoral success when she won only the party's second UK council seat in Oxford.
That was followed by election to the European Parliament in the South East England Region in 1999 - a seat she held until giving it up after her election to Westminster.