By Kylie MacLellan and Elizabeth Piper
LONDON (Reuters) - The leader of Britain's main opposition Labour Party urged voters on Thursday to back a campaign to remain in the European Union "warts and all", but his appeal did little to conceal his lingering euroscepticism.
Underlining the challenges for a referendum that is blurring Britain's traditional political divisions, Jeremy Corbyn spent more time attacking Prime Minister David Cameron, who is leading efforts to remain in the EU, than praising the bloc.
Corbyn used his first official speech on the EU to call on voters to participate in the referendum on June 23, trying to spur enthusiasm among pro-EU, left-leaning young people who may hold the key for the "In" campaign.
"You cannot build a better world unless you engage with the world, build allies and deliver change. The EU, warts and all, has proved itself to be a crucial international framework to do that," he said.
But the veteran left-wing lawmaker gave only scarce evidence that he had truly converted to the European cause, saying he was still critical of its shortcomings. These included a lack of democratic accountability and pressure to privatize public services, he said.
"Many people are still weighing up how they will vote in this referendum. And I appeal to everyone, especially young people...to make sure you are registered to vote. And vote to keep Britain in Europe this June," he told Labour members.
He said he had voted against membership of the then European Economic Community, the forerunner to the EU, in a 1975 referendum, but had been swayed by members of his party and the trade unions to now support staying in the bloc. But it must do more to protect workers' rights, he said.
"Overwhelmingly the Labour Party and trade unions have come to the view that they want to campaign for a social, just Europe...That's the party that I lead and that's the position I'm putting forward."
While more than half of Labour voters favor "In", opinion polls suggest that Britain as a whole is evenly divided over whether it should stay in the European Union.
With older voters, who largely support leaving the bloc, more likely to turn out, the "In" campaign wants to spur enthusiasm among younger Britons, many of whom backed Corbyn as Labour leader last year on a groundswell of desire for change.
With Cameron's popularity suffering as he struggles with questions over his personal wealth, a steel industry crisis and divisions in his ruling Conservative Party, Corbyn is for the first time in weeks seen as a safer pair of hands.
The YouGov poll published on Thursday found that public trust in Cameron had fallen 8 percentage points to 21 percent since February, while Corbyn's rating had risen 2 percentage points to 28 percent.
Corbyn attacked the prime minister's failure to save the ailing steel industry and to bring tax dodgers to justice.
"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, underlining the differences with Cameron in his approach to the EU.
Nevertheless, Cameron welcomed Corbyn's speech, saying the referendum overcame traditional political differences.
"The truth is this there are lots of things we disagree about between Labour, Liberals, Greens and others but the fact is we all come together to support the idea of Britain staying in a reformed European Union," he said at an "In" campaign office.
"Britain stronger in Europe," the campaign group to remain in the bloc, said Corbyn had made "a compelling case for our continued membership of the EU".
Corbyn initially refused to rule out campaigning to leave the bloc when he was elected leader last year. On Thursday, though, he denied he would be half-hearted in his push to remain in the bloc.
"You'll hear plenty from us on this. There's nothing half hearted about anything I do," he said.
(Additional reporting by William James and Estelle Shirbon, writing by Elizabeth Piper, Editing by Angus MacSwan)