European Union leaders swept aside U.K. opposition and nominated Jean-Claude Juncker to the EU’s top job, threatening to further fray British ties with the bloc ahead of a planned referendum on the country’s EU membership.

Government heads meeting in Brussels proposed Juncker, a 59-year-old former Luxembourg prime minister, for European Commission president after he ran for the post as the candidate of Europe’s Christian Democratic parties in EU-wide elections. U.K. Prime Minister David Cameron was out-voted by his fellow leaders, the first time such a decision wasn’t taken unanimously.

“I’ve told EU leaders they could live to regret the new process for choosing the Commission president,” Cameron said in a Twitter posting. “I’ll always stand up for U.K. interests.”

Juncker’s nomination, which must be approved by the European Parliament, underscores the opposing forces driving the 18-nation euro area toward greater integration and Britain toward the exit. Cameron has pledged to renegotiate Britain’s relationship with the EU before an in-or-out referendum in 2017 if he wins next year’s national election.