LONDON — Until now, Jeremy Corbyn, the leader of Britain’s opposition Labour Party and a longtime critic of the European Union, has said relatively little about the coming referendum on the country’s membership in the 28-nation bloc.

But on Thursday, he urged Britons to vote to stay in the union, “warts and all,” describing it as a crucial tool against climate change and tax evasion, and saying there was a “strong socialist case” for remaining.

The intervention is significant because Mr. Corbyn, a left-winger, has won the support of many younger people, including students, who tend to vote in fewer numbers than older citizens.