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LONDON—U.K. Prime Minister David Cameron sought to inject fresh energy into his campaign to keep Britain in the European Union, after some recent polls suggested momentum was growing for the campaign to leave the bloc ahead of a referendum this month.

Mr. Cameron's intervention Sunday came after two polls put the pro-leave campaign in the lead ahead of the June 23 vote, although the majority of surveys still give the pro-EU side a slight advantage.

He warned that a vote for a British exit, or so-called Brexit, would hurt the U.K. economy, leaving the government with less money to spend on pensions and health care than it would have if the U.K. remained an EU member.

Speaking in an interview with the British Broadcasting Corp., he repeatedly underscored the risks of leaving, saying the U.K. would face years of uncertainty outside the bloc while it negotiated new trade deals. In a flurry of media activity, the prime minister also gave an interview to one newspaper and penned an article setting out some of the risks of leaving for another. Separately, Treasury chief George Osborne warned in a newspaper article Sunday that leaving the EU would likely lead to a squeeze on defense spending in Britain.

"If we vote 'out,' it is a decade of uncertainty, and we shouldn't risk it," Mr. Cameron said.

Those campaigning for Brexit have accused Mr. Cameron of scaremongering, and say Britain's economy would thrive outside of the 28-member bloc. "We are divorcing ourselves from a failing political union so we can engage with a bigger, broader world," Nigel Farage, leader of the UK Independence Party and a prominent Brexit campaigner, said to the BBC.

A poll published late Friday by ORB International put support for leaving the EU at 53% against 47% for remaining, rising to 55% against 45% once adjusted for a projected higher turnout among Brexit supporters. The survey excluded those who were undecided. The findings sent the pound tumbling in currency markets Friday, falling 1.4% on the day to $1.4257.

A separate poll published Sunday by YouGov PLC put support for leaving the EU at 43% and those favoring remaining at 42%, with the rest undecided or unsure how they would vote.

John Curtice, professor of politics at the University of Strathclyde in Scotland and a leading authority on polling, said the latest polls signal that momentum appears to be shifting slightly in favor of the pro-Brexit campaign, but he added neither side has yet opened up a convincing lead.

"The balance of evidence is there's been a small movement toward leave," Mr. Curtice said, adding that the change was still too small to signify a turning point in the polls.

A run of polling results in May had appeared to show advances by pro-EU campaigners, with some surveys suggesting a lead of several points or more. But that gap in the polls has narrowed this month.

The average of the past six polls puts each side on 50%, according to NatCen Social Research, a nonpartisan social research agency, which has been tracking referendum polls.

Mr. Cameron on Sunday defended his leadership of a campaign that critics say has laid too much emphasis on the risks of leaving and not enough on the benefits of EU membership.

The prime minister said there is a "strong, bold and patriotic" case for remaining in the EU, which he said has strengthened the U.K.'s power and influence. He said the remain camp intends to underline the benefits of membership for jobs and young people in the coming days. He added that a public vote in favor of continued membership will give the U.K. extra clout within the EU.

Mr. Cameron played down the significance of the latest polls, but there are signs the pro-remain camp is stepping up its attacks on its opponents.

In a televised debate Thursday, pro-EU energy minister Amber Rudd made a personal attack on leading Brexit campaigner Boris Johnson, saying that while he's "the life and soul of the party" he isn't the man you would choose to drive you home. She also accused Mr. Johnson of seeking Mr. Cameron's job, a charge he denied in an interview published Sunday by the Sunday Times newspaper.

Following the debate, Mr. Cameron praised Ms. Rudd on Twitter, saying she made a clear and passionate case for EU membership.

Jenny Gross contributed to this article.

Write to Jason Douglas at jason.douglas@wsj.com

(END) Dow Jones Newswires

June 12, 2016 21:05 ET (01:05 GMT)

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