The Green Party in England and Wales has voted to change its rules so it has just one leader in future.
The decision, backed by 73% of members who took part in a ballot, ends the previous system of having two "principal speakers".
Caroline Lucas, a current principal speaker, said this would mean "strengthening our effectiveness".
Some Greens had said that having a single leader would compromise the party's principles.
About 7,000 Green Party members were balloted, with just over half taking part.
Of these, 27% were against the idea of a single leader.
Ms Lucas said: "I'm delighted about this result. The party can now move forward together and onto the job in hand. We have an urgent green message to communicate and many votes to win.
"This is a fantastic day for the Green Party and will help ensure we have a party that is understandable, recognisable and effective.
"But we now need to demonstrate to all our members, regardless of which way they voted, that this is not about weakening our principles, it's about strengthening our effectiveness."
Ms Lucas and Derek Wall currently share the role of principal speaker, as party rules say there must be one man and one woman in the job.
Mr Wall did not back the reform plan.
He is a prominent member of the Green Empowerment Campaign, which has outlined 10 "good reasons" to avoid having one person in charge.
Among its criticisms is that a single leader "would have no constitutional powers so would have little respect from those inside or outside the party who expect a leader in the conventional sense of the word".
"Electing the 'wrong' leader could be a disaster," the campaign said.
"For instance, someone with no charisma, a loose cannon, out of line with policy, inflexible, reinforcing stereotypes, having their own agenda, or worse," it added.
While environmental issues have been prominent recently in the policies of most political parties, the Greens have yet to make a breakthrough at Westminster.
There was an increase in councillors in local elections in May, but at the same time the Green Party in Scotland lost parliamentary seats at Holyrood.