David Cameron's hopes of securing a deal on reforms to Britain's relations with the EU were mired in delay as European leaders dug in their heels on a series of contentious issues in Brussels.
The delay looked set to scupper plans for the Prime Minister to return to London in time to chair a special Cabinet meeting on Friday evening to endorse the agreement and confirm a date for the referendum.
With European Council president Donald Tusk locked in one-on-one negotiations with several leaders, EU officials repeatedly postponed the meeting at which all 28 were due to assemble around the table to thrash out the final package.
What was initially billed as an "English breakfast" meeting was gradually put back to "English brunch" and "English lunch" until it morphed into something more like high tea at 4pm, Brussels time.
Mr Cameron 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.
Other leaders indicated the stark differences expressed around the table on Thursday on issues like migrants' welfare and relations with the eurozone may be narrowing.
Polish Europe minister Konrad Szymanski said a deal was "close", telling reporters: "We managed to have a compromise on many, many issues ... but we still need more clarification, more guarantees to get the compromise which would be satisfactory for both sides. There's still some way to go, we need some hours."
Along with other eastern European countries, Poland was resisting a British demand for an "emergency brake" on in-work benefits for migrant workers to be in place for as long as 13 years.
France and Austria voiced concern that protections for non-euro states proposed by Mr Cameron might hobble the development of the single currency.
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".
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.
One UK official said: "Nothing is agreed until everything is agreed ... What matters is that we get the right deal, it doesn't matter how long it takes."
The official added: "We are still here, we are still talking, so in that sense we are moving forwards. Is there a specific deal on a specific area? No."
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 admitted that after a long night of talks ending shortly before dawn there was "still no deal".
The Prime Minister pledged: "I will only do a deal if we get what Britain needs, so we are going to get back in there, we are going to do some more work and I'll do everything I can."
The two-day summit was initially scheduled to end at lunchtime on Friday but it became clear on Thursday evening that significant differences remained between leaders in a number of areas, with Downing Street describing talks as "hard-going".
As well as protection for non-euro states and the emergency brake, Mr Cameron was facing objections to proposals to cut child benefit for migrant workers' offspring overseas, exempt Britain from the requirement for "ever-closer union" and rewrite treaties to enshrine elements of the deal in the Union's fundamental law.
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."
Mr Schulz said he had the impression "the different positions are moving towards one another" and hoped that negotiations would come to an end on Friday, but he warned any package agreed by leaders would be subject to "intensive" consideration by MEPs at the European Parliament.