David Cameron is calling for more comprehensive reforms from Brussels as his bid to renegotiate Britain's membership of the European Union enters a crucial phase ahead of a crunch summit next month.
After talks with senior figures in Brussels, Mr Cameron said a proposed "emergency brake" on European Union citizens claiming benefits in the UK was"not good enough" and that the package "needs more work".
A deal at the February 18-19 summit is seen as vital if Mr Cameron wants to hold an early referendum on EU membership, because an agreement at a later date would make it hard to schedule a vote before the school summer holidays.
The Prime Minister said he could not be "certain" that a deal would be reached with the 27 other EU leaders at the meeting and stressed that he would only reach an agreement that addressed his concerns.
Number 10 said "significant progress over the coming days" would be required.
European Council president Donald Tusk, who will chair the summit, is due at Downing Street for dinner on Sunday to finalise "concrete" proposals which are expected to be published on Monday.
During a hastily-arranged trip to Brussels, Mr Cameron held talks with European Commission president Jean-Claude Juncker and European Parliament president Martin Schulz in an effort to keep his reform agenda on track.
Mr Cameron had demanded an outright four-year ban on migrants from other EU countries claiming in-work benefits in the UK but the compromise mechanism proposed would apply across all 28 member states.
There were signs of trouble in reaching an agreement however as Poland's foreign minister Witold Waszczykowski indicated his country would not back the brake mechanism.
It is thought that the proposed measure 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.
Migrants from the EU would then be barred from claiming in-work benefits for up to four years.
Crucially, it is believed that the decision on triggering the brake would be in the hands of national governments rather than EU officials.
Ukip leader Nigel Farage rejected the Prime Minister's claim that access to benefits acts as a draw for migrants coming to the UK.
He said: "The progress isn't very good, is it? We get an emergency brake that we have to ask permission to use.
"He will go to the summit in February like Oliver going up to the table and saying, 'Please Sir, can we have some more concessions?'. It's all pretty thin gruel."
After the talks, Mr Cameron called Francois Hollande to discuss the state of play in the negotiations.
Number 10 said he told the French president that "a deal in February remained possible if significant progress can be made in the coming days".
A spokeswoman added: "The Prime Minister said that if the deal on the table for February wasn't right, he wouldn't take it - there is no hurry and it is more important to fundamentally reform the UK's relationship with the EU."
A ComRes opinion poll for the Daily Mail found voters in favour of the UK staying in the EU by 54% to 36%, with 10% undecided.
ComRes interviewed 1,006 adults by telephone between January 22 and 24.