David Cameron has accepted that he will not get agreement on his EU renegotiation at this month's Brussels summit.
The Prime Minister said the scale of his demands for fundamental reform of Britain's relationship with Brussels meant it would not be possible to get an agreement "in one go".
Following talks in Sofia with Bulgarian prime minister Boyko Borissov, Mr Cameron said he would use the final summit of the year to "keep up the pace of negotiations".
"There are significant reforms that week are seeking. We need fundamental, legally binding and irreversible changes," he said.
"The scale of what we are asking for means we will not resolve this easily. We need time to ensure that each issue is properly addressed because what matters most is getting the substance right.
"This is a large, bold and wide-ranging agenda and it is difficult. We are not going to agree it in one go so I do not expect to reach agreement at this December summit but we won't take our foot off the pedal.
"We'll keep up the pace on negotiations and we will use this summit to focus minds and to work on solutions in the toughest areas because we do need reform in each and every area I have set out."