David Cameron has begun talks with Donald Tusk as they seek to finalise an EU renegotiation package that could be backed by the rest of the bloc.
The European Council president arrived at Downing Street for a working dinner to thrash out the details of proposed reforms which he is expected to publish within days.
A deal at the next summit on February 18-19 is seen as vital if Mr Cameron wants to hold a spring referendum on EU membership.
Top of the agenda will be changes to strengthen a proposed "emergency brake" on European Union citizens claiming welfare in the UK - which Mr Cameron will demand must come into force immediately after an in/out referendum.
The measure 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 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".
"People understand they must create 'victory' out of whatever they are handed and in this case we think there has been a long series of humiliating capitulations leading to this point," he told the Murnaghan programme on Sky News.
"It is not going to answer the concerns of the British people. We need the power in our own parliament to determine what our migration policy is."
Shadow foreign secretary Hilary Benn told Murnaghan: "To have brought the whole future of our relationship with the European Union down to this one issue shows that the Prime Minister, I think, is missing the big picture.
"The idea that you would say 'Well, if I don't get just this one thing in the perfect form I am seeking, then we are off' is not actually the leadership we should expect from our Prime Minister".
He dismissed suggestions the party leadership was "lukewarm" on the question, insisting the Opposition remained "firmly in support of Britain remaining in the European Union".
It is thought that the proposed benefit ban would be available to all EU states, and be activated when migration levels were deemed high enough to put public services or welfare systems under severe strain.
It is one of four areas where he 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 is 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.