EU leaders have returned to the negotiating table after 20 hours of behind-the-scenes talks as David Cameron and European Council president Donald Tusk struggled to keep Britain's renegotiation on track.
The 28 leaders had initially been due to gather early in the morning for an "English breakfast" meeting to approve a package of reforms to the UK's membership, but breakfast became brunch, lunch, high tea and then dinner as opponents of the deal dug in their heels.
The delays forced Mr Cameron to scrap plans to summon ministers for a Cabinet meeting on Friday evening to endorse the deal and fix a date for the in/out referendum - widely expected on June 23.
The meeting would also have given the green light to Eurosceptic ministers like Iain Duncan Smith and Chris Grayling to go out and campaign for Britain to leave Europe, and put pressure on waverers like London mayor Boris Johnson and Cabinet minister Michael Gove to spell out where they stand.
Sources close to Mr Gove declined to comment on increasingly widespread reports that the Justice Secretary is preparing to throw his weight behind Brexit.
Acknowledging his plans had been delayed, Mr Cameron said in a message on Twitter: "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."
After talks which stretched through Thursday night, Mr Tusk resumed one-to-one discussions with a series of leaders, including Mr Cameron, over the course of Friday, in the hope of forging a deal acceptable to all 28 members.
By the time the leaders gathered at 8.30pm Brussels time, it was not clear whether he had found a form of words which would satisfy all of them.
Mr Cameron faced concerns from eastern European countries like Poland, Hungary, the Czech Republic and Slovakia over his call for an "emergency brake" on in-work benefits for migrant EU workers to extend for as long as 13 years.
And the same nations put up stiff resistance to the UK's demand to impose cuts in child benefits for offspring living abroad on 34,000 existing claimants as well as future migrants.
Meanwhile, France and Austria voiced anxiety that the protections for non-euro states sought by Mr Cameron might effectively grant special status to the City of London and allow Britain to hobble future deepening of the eurozone.
President Francois Hollande said France wanted ''a financial regulation system which is valid in all parts of Europe'' with no ''right of veto'' for individual countries, while Austrian chancellor Werner Faymann said the UK must not be allowed to exercise ''a blockade against genuine banking union or other deepening of the eurozone''.
Speaking on France Inter radio, Mr Hollande said he had floated a proposal for the eurozone to be given its own government, parliament and budget to make it "more dynamic", whether Britain stayed in the EU or not.
"I will do what it takes for the UK to stay in Europe, but it is also necessary for Europe to be able to advance," said the French president.
"There can be no special status for the UK and the City of London.
"The UK's interests must be taken into account, but not to the detriment of Europe."
He warned that, if Britain left the EU, other member states might consider doing the same, adding: "We must avoid national withdrawals."
Belgium insisted on a "no second chance" clause in the summit conclusions to make clear that the UK would not be offered a better deal if it voted to leave the EU, in the hope of reversing the decision in a second referendum.
And there was opposition to Mr Cameron's call for Britain's hoped-for exemption from the requirement to seek ever-closer union, as well as changes to the relations between euro 'ins' and 'outs' to be enshrined in law by incorporation into the EU's treaties.
In a surprise move, Greece's Alexis Tsipras threatened that, unless other countries promised not to close their borders to the refugees flooding into his country, he would withhold his approval from the official summit conclusions - effectively vetoing not only the UK package but also an agreement on migration hammered out in the early hours of the morning.
Arriving at the summit venue after just three hours of sleep on Friday morning, Mr Cameron promised to do ''everything I can'' to finalise a ''credible'' package of reforms. But he insisted he would not sign up to an agreement which fell short of Britain's requirements.
He told reporters he was ''happy to stay until Sunday'' if need be, adding he had warned wife Samantha and their children he may not be home this weekend.
As the day wore on, Mr Cameron had meetings with Mr Tusk, German Chancellor Angela Merkel and Polish PM Beata Szydlo, as well as his Danish, Czech and Dutch counterparts.
But Czech Europe minister Tomas Prouza suggested that the UK side was not engaging in negotiations, writing on Twitter: ''As the time passes, I am more and more perplexed by the British approach of non-negotiation. Quite unorthodox, to say the least.''
In a swipe at the PM, European Parliament president Martin Schulz said: ''We must make clear that the method 'I will tell you what you must do in order for me to stay' doesn't work.
''We have to reach out to one another. I have the impression that David Cameron sees that.''