British Labour Party leader Jeremy Corbyn made his first major speech against leaving the European Union on Thursday, as analysts said his intervention could help to boost the turnout of Labour supporters in a referendum on June 23.
Corbyn appealed to voters, especially younger voters, to take part in the referendum and reject a British exit, or Brexit, from the EU.
He said membership of the EU had brought trade and investment opportunities and helped to protect Britain's employment rights and broaden human rights.
"The Labour Party is overwhelmingly for staying in because we believe the European Union has brought investment, jobs and protection for workers, consumers and the environment, and offers the best chance of meeting the challenges we face in the 21st century," he said.
Corbyn, who leads the official opposition party to Prime Minister David Cameron's Conservatives, had faced criticism over his low profile in the Brexit debate.
"Labour is convinced that a vote to remain is in the best interests of the people of this country," he told an audience at the University of London.
"But we also need to make the case for reform in Europe, the reform David Cameron's government has no interest in, but plenty of others across Europe do," Corbyn said.
"There is a strong socialist case for staying in the European Union, just as there is also a powerful socialist case for reform and progressive change in Europe.
"Many people are still weighing up how they will vote in this referendum," he said.
"And I appeal to everyone, especially young people - who will live longest with the consequences - to make sure you are registered to vote."
Many analysts fear a low turnout would favour the Leave side, which has a strong door-to-door "ground" campaign led by the UK Independence Party.
"A lot of people are saying it's going to be about turnout," Simon Hix, a political scientist at the London School of Economics, told reporters on Thursday.
"It's going to be about getting those Labour voters and giving them a reason to come out and vote, and giving them a reason to vote that's not a vote for Cameron," Hix said.
"What he's saying is something very different from the Cameron message," Hix said. "This is [Corbyn] directly appealing to younger voters on the left, to persuade them why they need to come out, give them a reason to come out and vote."
Some Labour members claim Corbyn is backing the "In" vote to promote unity in his party, despite his previous strong criticism of the EU.
"A lot of Labour supporters will worry that he's putting the party before the country," Kate Hoey, a Labour member of parliament who supports the Leave campaign, told the BBC.
But Chuka Umunna, a pro-Remain Labour member of parliament in London, said Corbyn had given "a great speech" on Thursday.
"Delighted with Jeremy Corbyn's speech making a powerful, progressive case for the UK's membership of the European Union," Umunna said on Twitter.
Cameron announced the referendum after EU leaders agreed a package of reforms that he said will give Britain a "special status" in the EU.
His main political opposition to Britain remaining in the EU comes from right-wingers inside his own party and from the anti-EU UK Independence Party.
In Washington on Thursday, International Monetary Fund chief Christine Lagarde described the British-EU relationship as "a long marriage."
"It's really my personal hope that that doesn't break," Lagarde said at a press conference ahead of IMF-World Bank meetings.
World Bank President Jim Yong Kim said the referendum was "an issue for the British voters," but he cautioned that Britain's economy "is not going to do well with more uncertainty."