A European Union summit to negotiate new membership terms for Britain was forced into extra time on Friday as Prime Minister David Cameron struggled for a deal he could sell to skeptical British voters in a referendum.
After all-night negotiations followed by a day of private meetings to try to narrow remaining differences, a senior EU official said talks were at a "critical" juncture.
A plenary session to review progress was postponed several times - from a late "English breakfast" to an "English lunch" and again till dinner at 8 p.m. (1900 GMT) - and then the 28 leaders were asked to book hotel rooms for an extra night in Brussels.
Cameron canceled plans to fly home and chair a cabinet meeting due to have approved a deal and set in motion procedures to call a plebiscite expected on June 23.
"Negotiations are continuing into this evening. A cabinet meeting won't be possible tonight. One will be held if and when a deal is done," the prime minister said in a Twitter message.
EU officials and diplomats said the main outstanding problems involved Britain's demands to curtail welfare benefits for migrant workers from other EU countries.
French President Francois Hollande, who raised objections earlier to British demands to protect the City of London financial sector from euro zone regulation, said the leaders were in for a "long evening".
"We will find a compromise," he told France Inter radio, adding: "I hope so."
German Chancellor Angela Merkel did not wait for the much delayed dinner, strolling out with aides to a renowned kiosk on a Brussels square to savor a cornet of Belgian "frites" (French fries) with spicy "andalouse" mayonnaise.
European Parliament President Martin Schulz, whose assembly will have to pass legislation to implement concessions to Britain on curbing EU workers' benefits, said the situation at the summit was "really tense".
"I am still optimistic that we find a solution but everybody has to move. Also the British government has to move," he told reporters, criticizing some countries for trying to link demands on Europe's refugee crisis with a British deal.
Resistance
The extension appeared due to last-minute resistance by east European countries to Cameron's efforts to cut child benefits for EU migrant workers whose children stay in their home country - a measure others such as Denmark are eager to emulate.
Greece also raised a potential hitch, saying it could block the entire deal unless it got its way on a dispute with Slovenia over border controls to curb the flow of migrants. But diplomats said the move was not considered a serious show-stopper.
One diplomat said the day of talks appeared to have largely settled concerns in France about differential treatment for London banks outside the euro zone and Belgian grumbles about Britain setting a precedent for states to snub EU integration.
The stakes are high for both Britain and the EU, with opinion polls showing voters almost evenly split.
The risks of Cameron's strategy were highlighted on Friday when an opinion poll showed the campaign to leave the bloc had a two-percent lead with 36 percent support. The TNS poll showed 34 percent of British voters wanted to stay in the bloc, 7 percent would not vote and 23 percent were undecided.
Summit chairman Donald Tusk held a series of so-called "confessional" meetings with individual leaders throughout the day to try to clear remaining obstacles.
Diplomats said differences with France, which was determined to prevent Britain gaining any veto on future euro zone integration or competitive advantage in banking regulation, had been narrowed down to just two words and would be fudged.
Cameron has promised Britons he will exclude new European immigrants from in-work benefits for four years and cut child benefit for workers whose families remain at home.
Czech Prime Minister Bohuslav Sobotka, representing that group, was battling to prevent the measures being applied to more than a million EU workers already in Britain and to avoid other countries piggy-backing on the child benefit cut.
Poland's Europe minister, Konrad Szymanski, said progress had been made and the proposed mechanism would be limited to Britain and "proportionate and shaped in the right way".
However, Danish Prime Minister Lars Lokke Rasmussen said his country too was keen to apply a plan to index child benefit for EU workers whose children remain in their native country to their home country's cost of living if Britain won.
Rasmussen claimed paternity of the idea, telling reporters: "It was a flower in my garden."
Cameron was keen to show British voters he was fighting hard to secure a deal which he has called "the best of both worlds".
Semi-detached
Britain is already the EU's most semi-detached member, having opted out of joining the euro single currency, the Schengen zone of passport-free travel and many areas of police and judicial cooperation.
Many leaders said they felt they were at a historic turning point for European integration.
No country has ever voted to leave the Union. Britain is the EU's second-largest economy and one of its two permanent members on the UN Security Council. Its exit would end the vision of the EU as the natural home for European democracies and reverse the continent's post-World War Two march toward "ever closer union".
Belgium, the most federalist of EU members, was pressing for a clause to ensure the deal with Britain would automatically cease to exist in case of a vote to leave - to make sure there was no possibility of a second renegotiation.
Diplomats said the idea was attractive to Cameron, who is seeking to convince Britons this is a last-chance vote, and might well be included in the final text.
The issue has divided Cameron's Conservative Party for decades, crippling his 1990s predecessor John Major and bringing down his hero Margaret Thatcher.
Some Conservatives have criticized the reforms he is negotiating in Brussels as trivial, although most senior party figures are likely to join him in campaigning to stay in if he wins the concessions he is seeking.
Britain's largely euroskeptic press depicted Cameron as begging or pleading, the Daily Mail describing him as "rattled".
"Shambles as embattled PM's deal is watered down," a front-page headline read over a picture of an anxious-looking Cameron.
