The UK should remain in a "fairer, kinder, more sustainable Europe", Green party leader Natalie Bennett has said.
She told a party conference in Harrogate that arguments for staying in the EU were "powerful" and "sensible".
These included free movement of people, which "enriches all of our lives", workers' rights and environmental standards, she said.
Prime Minister David Cameron has called an in/out referendum on the UK's relationship with the EU for 23 June.
Ms Bennett spoke of the mandate she had received from attendees at a party conference last year "to run a strong, bold campaign calling for Britain to remain in the EU".
"And that's just what we'll be doing," she said.
One senior party figure, Baroness Jenny Jones, has said on Twitter that she will vote to leave the EU.
Ms Bennett also told her audience it was "critically important that over the next four months our politics is not dominated by the European referendum to the exclusion of the pressing economic, social and environmental problems that Britain faces.
"We cannot, we must not, let David Cameron and his friends in the still out-of-control financial sector, in the oil and gas industry, in the tax-dodging multinationals, continue on the current path under the cover of the EU referendum. We must not be distracted."
Mr Cameron is campaigning for Britain to stay in the EU, which he says will be better for jobs and national security.
Former Conservative leader Lord Howard earlier became the latest senior Tory figure to disagree, saying the EU was "flawed and failing" and Britain should vote to leave.