David Cameron has dismissed criticism of a proposed "emergency brake" on benefit claims by EU migrants as he prepares for crunch renegotiation talks in Brussels.
The Prime Minister insisted the mechanism being discussed with the European Commission was "totally different" from previous ideas mooted, saying he was "encouraged" by progress.
However, he stressed there was "still a long way to go" before a deal could be finalised and he was willing to be "patient".
The comments, in an interview with BBC Radio Scotland after visits north of the border, came as Mr Cameron heads for Brussels to meet Commission President Jean-Claude Juncker.
The premier said his "first challenge" would be to reach Belgium, as Storm Gertrude has been disrupting flights.
"My first challenge is to get to Brussels because with Hurricane (sic) Gertrude I have already had a flight problem," Mr Cameron said.
"So I am racing across Scotland at the moment to get to Edinburgh Airport. But I hope I will make it."
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.