EU leaders were hunkering down Friday for a new round of efforts at a reform deal that is meant to keep Britain in the bloc, amid reports that consensus was proving difficult to achieve on the key issues of contention.
The stakes are high, with the European Union hoping that a reform package will convince Britons that they should vote for their country to stay a member, in a referendum that is widely expected to take place this year.
But the Brussels talks have run into overtime, with leaders insisting that agreement cannot come at any price.
"I was here until 5 o'clock (0400 GMT) this morning working through this, and we've made some progress, but there's still no deal," British Prime Minister David Cameron said upon his return to the summit venue, less than six hours later.
"I'll do everything I can," he added, stressing that he would only agree to a deal "if we get what Britain needs."
"The proposals currently on the table don't satisfy all the parties," added Luxembourg Prime Minister Xavier Bettel. He expressed hope of finding an acceptable compromise by the end of the afternoon, but some diplomats said the talks could stretch into the weekend.
"It is not possible to have such a show again in the coming months or years, with a permanent doubt hovering over Europe," Belgian Prime Minister Charles Michel said. "Today is the time for a deal."
But leaders will first have to find a compromise between their refusal to budge on basic EU values and the reforms on sovereignty, immigration, economic governance and competitiveness that Cameron is demanding before he campaigns for Britain to stay in the bloc.
EU President Donald Tusk spoke Thursday of a "make or break summit," with fears rife that the country is heading towards Brexit - the buzzword coined for a British departure from the EU after more than 40 years of half-hearted membership.
Many worry that this would destabilize the EU and reduce the bloc's global influence, at a time when it is already struggling with a migration crisis and enduring economic woes.
"I understand that we all of course pursue our national interests, but we should also bear in mind that if Britain leaves [the EU] we all get nothing," Estonian Prime Minister Taavi Roivas said Friday.
Lawyers had worked through the night at drafting compromise texts, an EU source said, noting that the 28 leaders would only return to the issue once there was a clear perspective of reaching an agreement.
This is now expected to happen at around 4 pm (1500 GMT). The talks had initially been due to resume over breakfast.
In the meantime, small-group discussions were taking place, with a source saying around midday that "all the issues are still open."
"It does not look all that easy, but I remain an optimist," Austrian Chancellor Werner Faymann said as he arrived.
One of the crunch issues has been a British demand to curb welfare payments to European workers, with tensions in particular over child benefits for those whose families live outside Britain.
London is keen to limit the number of EU workers seeking jobs in Britain, amid a perception that continental Europeans are saturating its labour market and abusing its welfare system.
Just over 2 million EU citizens were working in Britain at the end of 2015, out of a total of around 31 million workers, according to national statistics released Wednesday. The country has the fifth-largest economy in the world.
Other unresolved issues relate to a British exemption to the EU's stated goal of "ever closer union," as well as a proposed mechanism for non-eurozone countries such as Britain to raise concerns about initiatives taken by the currency bloc.
Speculation has been rife that if a deal is reached at the Brussels summit, it will pave the way for Cameron to call the in-out referendum for June 23.
"We would like to see Britain in Europe and we would like to help British people to make a decision," Lithuanian President Dalia Grybauskaite said. "But no matter what we do here, no matter what face-lifting or face-saving we perform here, it's only up to the British people to decide."