Cameron is seeking changes to the U.K.-EU relationship that will let him urge Britons to vote “yes” to continued membership in a referendum that could come as early as June.

Prime Minister David Cameron faced off Thursday against the 27 other European Union leaders, telling them to grant his country a new deal to settle the festering issue of their relationship or face a possible divorce as soon as this summer.

Pressure mounted for a deal Friday at the end of a two-day summit, after initial talks dragged through the night and highlighted differences across the continent.

Cameron said he was “battling for Britain” at a Brussels summit – and for a less intrusive EU that would benefit other countries, too. But French President Francois Hollande warned that no individual leader should be allowed to stop closer European cooperation.

“It’s the EU in question, not just one country in the EU,” Hollande said as he arrived. “I want Britain to stay in the EU. But I hope most of all that Europe can advance, can be stronger.”

Cameron is seeking changes to the U.K.-EU relationship that will let him urge Britons to vote “yes” to continued membership in a referendum that could come as early as June.

He told his fellow leaders that he needed a substantial deal that would be “credible for the British people.” The British referendum on EU membership is bound to be hard-fought, since few issues in Britain have as much resonance as its relationship with the EU.

“The question of Britain’s place in Europe has been allowed to fester for too long,” Cameron said. He warned Britain would walk away if the deal was not good enough.

“If we can get a good deal, I will take that deal. But I will not take a deal that doesn’t meet what we need,” Cameron said.

The leaders discussed Britain’s reform aims before a six-hour dinner meeting focusing on the migrant crisis. When the politicians finally dispersed at 2:30 a.m. (0130GMT), EU President Donald Tusk said they had “made some progress but a lot remains to be done” to secure a deal for the U.K.

A British official said gaps remained on key issues. Bilateral talks and negotiations by diplomats and lawyers continued through the night before the 28 leaders reconvene for breakfast Friday.

Britain, which has one of the strongest economies in Europe, has been a magnet for hundreds of thousands of workers from eastern EU nations who are seeking higher-paying jobs. Britain has no power to stop immigration from other EU nations, leading some in Britain to say that immigrants are taking their jobs. The EU immigrants can also claim unemployment, child care and other benefits in Britain, which Cameron’s government says is straining the country’s social services budget.

Since none of the 27 other leaders wants to see Britain leave, there is broad consensus, if not agreement, on a deal which Cameron says he needs to win the referendum. It would give Britain more powers to limit benefit payments to workers from other EU countries for several years – something Britain says will slow the pace of immigration. The draft deal also offers guarantees to countries, including Britain, that do not use the shared euro currency, and makes tweaks aimed at boosting competitiveness and giving national parliaments more power.

But differences remain on key details.

One European official said a main source of tension was the length of time Britain’s limit on welfare benefits would last, with Cameron pushing for 10 years and Eastern European countries pushing for three or four. The official spoke on condition of anonymity, since the discussions were not public.

The lingering disputes belie the fact that the other member states cherish Britain as an economic and diplomatic giant in a struggling EU.

“I’m going into this debate with the position that we would like to do everything to create the conditions so that Great Britain can remain part of the European Union,” German Chancellor Angela Merkel said.

Cameron argues for a “live and let live” union in which some countries move ever closer together while others remain semi-detached. That grates against the wishes of France, Germany and others who believe strongly in an “ever closer union” among as many members as possible.

Britain has long been a half-hearted member of the EU, staying out of both the euro currency and the passport-free Schengen travel zone. The perception of increasing Brussels meddling in affairs many Britons considered sovereign issues made the time ripe for a U.K. referendum.

Cameron said he would not stop other EU members striving for more unity, but insisted Britain should have ironclad guarantees that it could stay on the sidelines.

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