A day of "intensive" negotiations over EU reforms is getting under way as David Cameron battles with Brussels to secure a reform package in time for an early in/out referendum.
European Council president Donald Tusk approved a 24-hour extension after the Prime Minister claimed a significant breakthrough over migrant benefits but failed to reach agreement on a number of other areas.
The Prime Minister had hoped to finalise a renegotiation of Britain's relationship with Brussels over dinner with Mr Tusk at Downing Street so that it could be put to other leaders tomorrow, ahead of a crucial summit in less than three weeks.
Downing Street said he had secured a crucial assurance that a proposed "emergency brake" on welfare payments to EU workers could be triggered on the basis of present levels of immigration.
But after less than two hours of discussions, Mr Tusk left No 10 saying "intensive work" would be required over the next 24 hours if a draft agreement was to be published on Tuesday.
A deal at the next summit on February 18 and 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.
A Number 10 spokesman said "much progress has been made" since the PM scrapped a trip on Friday to Scandinavia in favour of talks with European Commission president Jean-Claude Juncker and European Parliament president Martin Schulz.
"The commission have tabled a text making clear that the UK's current circumstances meet the criteria for triggering the emergency brake.
"This is a significant breakthrough, meaning the Prime Minister can deliver on his commitment to restrict in work benefits to EU migrants for four years," he said.
"But there are still areas where there is more to do and both agreed it was therefore worth taking the extra time to make further progress."
One critical stumbling block is the UK's demand the mechanism be allowed to remain in place "long enough to resolve the underlying problem" - perhaps as long as seven years.
Others include ensuring new rules to protect non-euro countries from being harmed by decisions taken by those using the single currency were "watertight" and included a concrete mechanism for member states to escalate concerns.
And Mr Cameron is also pushing for more substantive proposals to close "back-door routes to Britain which have enabled non-EU illegal migrants to stay in Britain in recent years".
Diplomats - known as Sherpas - will meet early on Monday in Brussels and work through the day.
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.
The PM agreed to accept it as a "stop gap" measure - on the condition he received assurances the existing pressure on the UK would meet the threshold to trigger it and it could remain in place.
The agreement would mean that the four-year ban could be introduced quickly if voters chose to remain in the UK at the referendum - though it would have to await the completion of "logistical and process work" in Whitehall.
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.
Matthew Elliott, chief executive of the pro-Brexit Vote Leave campaign, said: "The final outcome of these negotiations will be a set of deeply trivial reforms that won't bring powers back from Brussels.
"David Cameron has already set out the most he's asking for in a letter to Tusk last year, so it's clear just how low the bar has already been set.
"Even with the avalanche of spin that will accompany the final deal, the renegotiation won't take back control for the British public. The only way to do that is to Vote Leave."