David Cameron has arrived in Brussels for crunch talks on his renegotiation plan as proposals for an "emergency brake" on benefits claims by EU migrants suffered a setback.
The Prime Minister met European Commission president Jean-Claude Juncker as he sought to keep his reform demands on track ahead of a summit with EU leaders in February.
But there were signs of trouble in reaching any agreement on the emergency brake mechanism as Poland's foreign minister Witold Waszczykowski indicated his country would not back it.
It is thought that the mechanism would be available to all EU states, and be activated when migration levels were deemed high enough to put public services 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.
But at a news conference in Warsaw Mr Waszczykowski expressed his country's opposition to the plan.
"It will not accept a mechanism that denies social benefits to Poles living in the European Union," he said.
Before arriving in the Belgian capital Mr Cameron dismissed criticism off the proposal and insisted the mechanism was "totally different" from previous ideas mooted, saying he was "encouraged" by progress.
In an interview with BBC Radio Scotland Mr Cameron was confronted with a previous quote he gave describing an "emergency brake" as an "arcane mechanism within the EU that would probably be triggered by the European Commission and not us".
But he replied: "That was actually a totally different idea and a different sort of brake to the one that is being proposed now."
Mr Cameron said: "I have always said we are open to alternative ideas as long as they have equal force...
"We have made progress. It is encouraging that people like the European Commission are coming forward with ideas. But there is still a long way to go before we see something that we can actually agree.
"There is still a lot of hard negotiation, a lot of hard talking. But it is encouraging that what I was previously told was impossible is now looking like it is possible."
Mr Cameron said he would not "agree to something unless it has the force and the weight that we need to solve the problem that we have". "I am prepared to be patient. We don't need to have our referendum until the end of 2017," he said.
"But we are clearly seeing the EC responding to the issues that have been put on the table by Britain and that is encouraging."
Mr Cameron is expected to try to thrash out details of the brake with Mr Juncker, and will also meet European Parliament president Martin Schulz.
A Downing Street spokesman declined to discuss the details of the emergency brake proposals, telling a Westminster media briefing: "We are making progress. It is good if people are coming forward with ideas and options, but we are still going to continue our negotiations in order to get the best deal for the British people."