BRUSSELS (AP) — British Prime Minister David Cameron is bracing for a crucial European Union summit on Thursday, convinced he needs to clinch a deal to fundamentally reform the bloc to make sure a U.K. referendum on continued British membership can be won.

With as many as four core issues still outstanding going into the two-day summit of EU leaders, EU President Donald Tusk said on the eve of the meeting that "frankly, there is still no guarantee that we will reach an agreement."

The lingering disputes over anything from the reinforcement of national sovereignty to child benefits belie the fact that none of the other member states wants to see the departure of Britain, an economic and diplomatic giant in a struggling EU.

"We must not lose this. It would be a defeat both for the U.K. and the European Union, but a geopolitical victory for those who seek to divide us," Tusk wrote in a letter to the summit participants.

On the other hand, Britain has been an increasingly half-hearted member over the past decades, deciding to stay out of both the common euro currency and the borderless Schengen zone. And the perception of increasing meddling of Brussels into affairs many Britons long considered sovereign issues made the time ripe for a referendum on membership.

Still, Cameron is convinced that if the right reforms are approved during the summit, the 28-nation EU will be good for Britain and he will campaign for an in-vote in the referendum, which could come as soon as June.

He has relentlessly consulted with EU leaders over the past months to get close to a deal on reform many would have long held impossible. A British official said that Cameron spoke with Tusk late Wednesday and both "agreed that good progress had been made in all four areas of our renegotiation."

But Nigel Farage, leader of the U.K. Independence Party, said the changes the British leader was seeking were trivial.

"It's not worth a row of beans, whatever he gets," Farage told the AP in Brussels Thursday.

On Wednesday, German Chancellor Angela Merkel said that it was in Germany's national interest for Britain to remain in the EU.

In a speech to Parliament, Merkel painted a largely positive picture of chances of an agreement on Britain's EU reform demands, which she said are in many cases "justified and understandable."

"Germany will make its contribution so that a result that satisfies everyone can be achieved, if possible already at the summit," she said.

Merkel said Britain is an ally for Germany in promoting competitiveness and free trade, and that "Europe needs Great Britain's foreign and security policy commitment to assert our values and interests in the world."