But Donald Tusk, the president of the European Council, said the meeting on Dec. 17-18 should "pave the way for a deal in February", when the leaders are next scheduled to meet.
Speaking at a press conference in Sofia following talks with Bulgarian Prime Minister Boyko Borissov, Cameron said Britain was seeking "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."
He continued: "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."
Cameron’s office in Downing Street announced he had told German Chancellor Angela Merkel in a telephone call earlier Thursday that a December deal would not be possible.
In a landmark policy speech last month detailing renegotiation of Britain’s membership terms, Cameron had said that it was of "cardinal importance for the United Kingdom" that the EU recognized the rights of its members who did not use the euro.
In addition to new rules around the single currency, he said he wanted the EU to address three other issues: a greater say for national parliaments in EU affairs, a crisis of European competitiveness in the world and the need for greater migration controls.
But European leaders, including Merkel, have said they would not accept amendments to the principle of freedom of movement.
Cameron’s Conservative Party has promised to hold an in-out referendum on the U.K.’s membership of the EU before the end of 2017.
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