Donald Tusk said another day of "intensive work" is still needed to secure an EU renegotiation package after he emerged from talks with David Cameron declaring "no deal".
Number 10 had hoped to finalise a renegotiation of Britain's relationship with Brussels over dinner at Downing Street so that it could be put to other leaders tomorrow ahead of a crucial summit in less than three weeks.
Mr Cameron was seeking assurances that a proposed "emergency brake" on welfare payments to EU migrants could be triggered immediately after the vote in a bid to reassure critics it would be effective in slowing the influx of foreign workers.
But after less than two hours of discussions, Mr Tusk left No 10 after less than two hours and announced via social media: "No deal yet. Intensive work in next 24 crucial."
The PM said it had been "a good meeting with Mr Tusk, who has agreed to another 24 hours of talks before publishing the draft UK renegotiation text".
A deal at the next summit on February 18-19 is seen as vital if Mr Cameron is not to run out of time to put the future of the UK's membership to the public before the summer holidays.
The emergency brake has been put forward by Brussels as an alternative to Mr Cameron's plan to impose a unilateral four-year curb which other member states ruled out as discriminatory and in breach of the freedom of movement principle.
Senior government sources said the PM was now prepared to accept it as a "stop gap" measure - on the condition he received assurances it could be triggered by present levels of immigration and could remain in place "long enough to resolve the underlying problem".
Ahead of the meeting with Mr Tusk, the PM gathered his most senior cabinet colleagues - Chancellor George Osborne, Foreign Secretary Philip Hammond and Home Secretary Theresa May to discuss the state of play.
Mrs May is being wooed by pro-Brexit campaigners who hope she will take advantage of the freedom offered by the PM and become a figurehead for the "out" campaign.
Mr Cameron insists he will not do a deal "at any price" and is prepared to hold off with the plebiscite - which must be held by the end of 2017 - if he considers the deal on the table to be inadequate.
But he cancelled Friday's scheduled trip to Scandinavia to fly to Brussels for talks with European Commission president Jean-Claude Juncker and European Parliament president Martin Schulz in a bid to keep the process on track for a vote as soon as June.
Eurosceptics have dismissed the proposals on the table as "pretty thin gruel" that would do little or nothing to stem the flow of would-be workers arriving in the UK, notably from eastern European states.
Steve Baker, co-chair of the anti-EU Conservatives for Britain group, said Mr Cameron was engaged in a "synthetic" row with Brussels and dismissed the renegotiation as a meaningless "farce".
Migration is one of four areas where Mr Cameron is seeking changes to the UK's relationship with the EU, alongside the lifting of member states' commitment to "ever-closer union", measures to protect non-euro states and improve EU competitiveness and greater powers for national parliaments.
He had been expected to press for the current proposals in all four to be strengthened further as well as for action to tackle " back door" immigration into the UK and other "abuse" of free movement rules.
Number 10 said the menu for the dinner comprised a smoked salmon starter and a main dish of fillet of beef with vegetables followed by by pear and apple crumble for pudding.