Prime Minister David Cameron argued for much of the night in Brussels on Friday with European Union partners determined to limit concessions on offer to help keep Britain in the bloc.
Fellow leaders and diplomats said an agreement that would allow Cameron to return to London and launch a campaign to stay in the EU at a June referendum still seemed feasible by the end of a two-day summit on Friday, but some said the outstanding issues were proving tough to crack, holding up the process.
A late-night dinner lasting more than five hours that was devoted to renewed arguments over the response to Europe's migration crisis also meant that a plan for an "English breakfast" on Friday for all 28 leaders to try and hammer out a final deal was now set to turn into “brunch.”
"It might take longer than they think," Irish Prime Minister Enda Kenny said on leaving the summit center, where Cameron remained locked in overnight talks with top EU officials, French President Francois Hollande and the Belgian and Czech premiers.
Those three national leaders made the case on the various points of most resistance to a draft agreement brokered by summit chairman Donald Tusk, who told reporters: "For now, we have made some progress but a lot still remains to be done."
Cameron finally left after his last meeting shortly after 5:30 a.m. local time, stepping swiftly and saying nothing.
Earlier, he appealed to EU leaders to help him settle the question of Britain's European Union membership for a generation by agreeing a "credible" deal he can sell to the British public.
But aides voiced frustration at a lack of concessions by partners who are wary of Cameron's bid to side-step EU regulation and cut immigration: "I would say the going is tough, this could be a long night," a British official told reporters.
"While many countries were saying they want to help, they want to make sure they keep Britain in the EU, there wasn't much sign of how they are planning to do that in practice, not showing much room for maneuver," the official added.