By  Laurence Norman

BRUSSELS—U.K. Prime Minister David Cameron’s three-year campaign to reset his country’s relationship with the European Union comes to a head on Thursday when the bloc’s leaders gather for a two-day summit in Brussels to craft a final deal.

Ahead of the meeting, the final pieces of a potential deal seemed to be falling into place and German Chancellor Angela Merkel again voiced her understanding for Mr. Cameron’s demands.

However, EU diplomats pointed to continued disagreements on several important points and while most said a deal was attainable this week, they warned of protracted, difficult negotiations in coming hours.

U.K. government officials have said an agreement this week would open the way to a vote on Britain’s EU membership in late June. Mr. Cameron first laid out in January 2013 his plan to win changes to the U.K. relationship with the bloc as a precursor for a British referendum on the EU.

Polls show British voters remain divided on membership. Mr. Cameron has faced a rocky ride at home since European Council President Donald Tusk on Feb. 2 published EU proposals for addressing his demands. Some members of his own party and others have argued that the proposed EU fixes, negotiated with London over months, are neither sufficiently broad nor watertight to represent a major change in U.K. ties with the bloc.

Mr. Tusk is expected to circulate modestly amended proposals Wednesday evening.

EU officials said the discussions between Mr. Cameron and his fellow leaders would start Thursday evening and could easily run late into the night. Discussions will very likely continue Friday morning in a push to reach a final deal.

“After my consultations in the last hours I have to state frankly: there is still no guarantee that we will reach an agreement,” Mr. Tusk said in a letter to EU leaders Wednesday afternoon.

Speaking to lawmakers in Berlin on Wednesday, German Chancellor Angela Merkel said she hoped for a final deal that would match Mr. Cameron’s concerns—including reducing welfare incentives for EU migrants to move to Britain—with the bloc’s desire to preserve key principles, including the freedom of movement for workers.

“We insist that the fundamental accomplishments of European integration are not put into question,” Ms. Merkel said. “Bringing into agreement this fundamental position with the British concerns is the problem that must now be resolved.”

Mr. Tusk’s proposals have addressed a range of U.K. concerns. To discourage workers to come to the U.K., the EU is offering an “emergency brake” for a number of years that would allow the British government to reduce in-work benefits to EU workers who arrive in Britain in the future. The U. K.—and other governments—would also be able to opt into a system that indexes child benefit payments to living costs for dependents of EU workers living overseas.

The EU has offered clarification of the call for an “ever closer union” to make clear it doesn’t commit the U.K. to political integration. Mr. Tusk is offering to strengthen the ability of national parliaments to block EU proposals; to limit EU red tape; and to protect key interests of the minority of EU governments who aren’t members of the eurozone.

However, key details need nailing down. On Wednesday, there were signs that one of the biggest open questions—how long the so-called emergency brake could be applied—was starting to be settled. A senior German government official said he could imagine that the plan could be applied for seven years.

However, the proposals need backing by all 28 governments.

EU countries must still agree whether the child benefits indexation should apply only to new workers who arrive in the future in Britain or those already there. The exact conditions for qualifying for the emergency brake remain under discussion with central and Eastern European governments eager to ensure the thresholds are as high as possible to stop other countries applying the brake.

Belgium is among the countries seeking changes to Mr. Tusk’s “ever closer union” proposal, with officials saying the current text strips the union of any effective unifying political goal.

The French and German governments are also pushing to ensure that the eurozone proposals, which include a mechanism allowing non-eurozone countries to raise up to EU leaders decisions that they think discriminate against them, won’t hand the U.K. a veto over their future plans. They also want to make sure that British regulators aren’t free to place lighter supervision rules on their banks in future.

“There must be a single rule book,” a French diplomat said Wednesday, although he added that agreement on these issues is “almost there.”

Meanwhile, Mr. Cameron is still pushing to win commitments to entrench at least some of the key concessions next time the EU’s basic treaties are revised.

Ms. Merkel said Wednesday her government remains open to that but others are reluctant. And even if Mr. Cameron wins that commitment, there are growing questions about how watertight that promise would be.

Mr. Tusk’s proposals don’t offer a clear legal text but instead, a series of clarifications and interpretations of existing rules. That means while other governments would be under what one diplomat on Wednesday called a “moral, political commitment,” to apply the principles, there would need be real negotiations in future on how they were converted into a future treaty.

Meanwhile, in her Bundestag speech, Ms. Merkel pointed to a fact that could undermine Mr. Cameron’s claim that a final deal would be irreversible and legally binding. She noted that any future treaty revision would need approval from the German parliament, a situation mirrored in several other EU governments.