Caroline Lucas has been elected as the first ever solo leader of the Green party after it switched from its system of having two principal speakers.
Ms Lucas, who is the MEP for South East England, beat the only other candidate, Ashley Gunstock, by 2,559 votes to 210.
In her acceptance speech, Ms Lucas said Green party support was growing.
There was an "amazing opportunity" for a party which "genuinely stands for social and environmental justice," she went on.
Addressing party members, she said: "More and more people are ready to vote for us and we need to be ready to provide them with the candidates and the party that they deserve."
Ms Lucas also criticised Gordon Brown for failing to impose a windfall tax on the profits on energy companies.
She told BBC News: "No other political party is really tackling the key issues of the day. We've got a Prime Minister who says that climate change is the greatest threat we face and yet there he is, unable to even levy any kind of windfall tax.
"He's unable to be really investing in renewables and energy efficiency. The opposition parties similarly are thrashing about. The Green Party is desperately needed today with real practical solutions to today's problems."
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