The Green Party has elected its sole MP, Caroline Lucas, as its new leader after she called for a "progressive alliance" with Labour.
Ms Lucas, the MP for Brighton Pavilion, will share the role with the party's work and pensions spokesman, Jonathan Bartley.
The pair have spoken of wanting to form a "progressive alliance" with Labour and others, not to stand candidates against each other to stop the Conservatives being elected.
The Green Party leadership contest was triggered when Australian Natalie Bennett announced in May that she was stepping down after four years in the job, saying she is not a "spin-trained, lifelong politician".
The results of the ballot were announced at the party's conference in Birmingham on Friday.
The Lucas/Bartley joint ticket was the overwhelming victor in the contest, taking 13,570 (88%) of the 15,467 votes cast.
In second place was David Malone with 956 votes, followed by David Williams on 527, Clive Lord on 173, Martie George Warin on 133 and Simon Ashley Cross on 108.
Ms Lucas addressed the UK's Brexit vote in her victory speech, telling party members that any deal struck by the Government with the European Union should be put to the British people in a second referendum.
"We cannot accept a deal that doesn't offer hope and security to both those who voted to Leave and those who voted to Remain," she said.
She also accused the Government of using EU citizens like "bargaining chips" as it prepares to negotiate with Brussels.
She insisted her party will continue to fight to protect the status of such individuals and to reassure them that they are welcome in the UK.
She said: "So, Conference, we give a guarantee today to the EU nationals who have made their home here: You are as much a part of Britain's future as we are.
"You help make our country great and we will continue to fight for your right to stay."
Standing alongside Ms Lucas for an acceptance speech in which they took turns to deliver passages, Mr Bartley confirmed that the pair will seek "progressive alliances" with other parties in the hope of overcoming the obstacle which the first-past-the-post system presents to smaller parties winning representation at Westminster.
But he did not make clear whether this could involve electoral pacts for parties to stand aside to give fellow progressives a better chance in their target seats.
"We are resolute in wanting to explore the potential for progressive alliances with other parties that will deliver fair votes, that will deliver more elected Greens than ever before," said Mr Bartley.
"Over a million people voted Green at the last general election. They deserve to have their views represented in Parliament. We owe it to them to be more ambitious than achieving just one MP."
He told activists: "A progressive alliance can mean different things in different constituencies. But it will not be top down and it will be up to you."
In a message to other parties, Mr Bartley said: "Old tribal loyalties are dying and voters can no longer be taken for granted. The era of two party politics is over.
"It's the voting system that is still stuck in the past. Change politics and we change people's lives forever ."
Electoral reform was at the heart of the "new political settlement" offered by the Green leaders, also involving a constitutional convention to consider an English Parliament, new civic assemblies and the devolution of power to cities and regions.
Mr Bartley said Greens would be "more united with two leaders than other parties are with one".
In a swipe at the Conservatives and Labour, he said: "If we do disagree, we talk about it and resolve the problem.
"We don't throw the country's security and stability away to settle an internal squabble.
"And we don't throw bricks through one another's windows.
"We are united in our passion for doing politics with people."
Borrowing the key slogan of the Leave campaign, Ms Lucas said that Greens would "give people real control" by nationalising the railways, reining in the banks, keeping the NHS out of private hands and reforming the energy system.
The co-leaders promised a "green industrial strategy" which would leave fossil fuels in the ground and make Britain a world-leader in renewable energy. They called for a "radical" redistribution of wealth, including a maximum pay ratio between the highest and lowest paid, the introduction of a universal basic income and protection for the welfare state.
Ms Lucas said the Brexit vote had left Britain "a divided country scarred in so many places by fear, inequality and hopelessness".
Greens will "resist the cruelty unleashed by the unbridled capitalism of May and Cameron" and "stand up for the majority currently locked out of politics and denied a voice", she promised.