A deal on the UK's demands for European Union reforms could be sealed in February after David Cameron was forced to accept an agreement could not be reached this month.
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".
European Council president Donald Tusk said a debate on Mr Cameron's renegotiation plans at a summit of leaders from the 28-member bloc later this month should "pave the way for a deal in February".
He will write to all EU heads of government on Monday with his assessment of the UK's proposals, he added.
British officials have been in close touch with Mr Tusk's team throughout the negotiation process.
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 we are seeking," he said.
"We need fundamental, legally binding and irreversible changes.
"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."
Earlier, Mr Cameron disclosed in a telephone call with German chancellor Angela Merkel - the EU's key power broker - he had concluded a December deal would not be possible.
The Prime Minister has submitted a series of demands for changes in the relationship with the EU ahead of the in/out referendum promised by the end of 2017.
A quick agreement would have given him more time to make the case for staying in and greater flexibility in choosing the referendum date.
Number 10 has insisted that Mr Cameron's call for a four-year curb on in-work benefits for newly arrived workers from the EU remained a "core part" of his renegotiation despite it being one of the main stumbling blocks in reaching a deal.
The Prime Minister is reported to have warned Mr Tusk and European Commission president Jean-Claude Juncker he is ready to campaign for Britain to leave the EU unless other leaders accede to his demands.
Asked about the report, a Number 10 spokesman said: "The issue on the four years remains a core part of our renegotiations.
"That certainly hasn't changed.
"The Prime Minister has always been clear that we don't rule anything out.
"His focus is very much on renegotiating the best possible deal for the UK and, once those renegotiations have been completed, taking that to the British people in a referendum."