British Prime Minister David Cameron arrived in Brussels on Friday for talks with European Commission President Jean-Claude Juncker, days before the EU is expected to issue its response to key reforms sought by London.
Cameron has outlined four areas in which he has demanded changes - competitiveness, sovereignty, social security and economic governance - before putting Britain's continued EU membership to an in-out referendum that he has promised to hold before the end of 2017.
Intense diplomatic efforts are under way to broker a deal between Cameron and his 27 counterparts at a summit of EU leaders scheduled for February 18-19.
But progress is slow and the issues at stake are thorny, officials in Brussels and London have acknowledged.
"There's still a long way to go before we see something that we can actually agree," Cameron told BBC Radio Scotland.
"There's going to be a lot of hard negotiation, a lot of hard talking, but it's encouraging that what I was previously told was impossible is now looking like it's possible," he added.
Cameron said he is prepared to wait for a deal that has the "force and the weight that we need to solve the problems that we have."
EU President Donald Tusk is expected to present his proposals responding to Cameron's four areas of reform early next week. This will give all 28 capitals a chance to discuss the measures ahead of the February summit.
It is far from clear, however, how other member states will respond or whether they can reach agreement next month, according to Jonathan Faull, who was appointed by Juncker to represent the commission in negotiations.
"Every member state will be looking at the legal implications for its people, its interests," Faull told EU lawmakers Wednesday. "I do not know what will come out of the [summit]."
Cameron's most controversial demand has been a restriction on access to in-work benefits for citizens of other EU countries until they have lived and paid contributions in Britain for four years.
One option on the table is a so-called "emergency brake" that could be applied if Britain can show that its welfare system is overwhelmed, British media have reported. Other member states would have to agree to the move.
"The question with these brakes and ideas, it's very important how they're pooled, how long they last, how much strength they have," Cameron said, adding that he would discuss these matters with Juncker.