Downing Street said "good progress" had been made on the UK's proposals but there were still "difficult issues" to resolve.
The prime minister's spokesman said there would be "substantive discussions" on the reforms instead.
The UK's PM has promised an in/out vote on EU membership by the end of 2017.
The prime minister is said to want to hold an early vote but he has said he will not set the timing of the poll until the negotiations, on Britain's terms of membership of the 28-member bloc, are concluded.
Q&A: What Britain wants from Europe
Guide to the UK's planned in-out EU referendum
BBC News EU referendum special report
Mr Cameron spoke to German Chancellor Angela Merkel by phone on Thursday to discuss the renegotiation efforts.
After the call, Downing Street said: "The prime minister explained that his priority is to get the substance right, underlining the need for legally binding, irreversible changes.
"He noted that the scale of what we are asking for means we will not resolve this in one go and consequently he did not expect to get agreement at the December European Council.
"Instead, we should keep up the pace of discussions and use the summit for a substantive discussion of the proposed changes in each area."
The government had never committed to a deal by December but it is thought that was what the negotiating team had hoped.
BBC assistant political editor Norman Smith said it suggested Mr Cameron was "running into real difficulties" in his renegotiations, in particular over proposals to restrict EU migrants' access to benefits in the UK.
"That has always been a sticking point," he said, adding that European leaders have "serious reservations" about the proposals.

The American screamed in agony after slipping during the third set against Romania's Sorana Cirstea on court 17.
The Women's Tennis Association (WTA) said more will be known following Friday's tests.
"Bethanie of course thanks everyone for their kind wishes and messages of support," the WTA statement added.
"She is resting tonight but wants to chat tomorrow [Saturday] via Facebook Live so she can let everyone know how she's doing and answer some questions."
Mattek-Sands was treated by paramedics on the court before being carried off on a stretcher and taken straight to hospital.
The 32-year-old later wrote on social media: "To the best family, friends and fans a girl could ask for, it's been a painful and emotional 36 hours but thank you for all the love and support."
Mattek-Sands had been favourite to add a Wimbledon doubles title to her US Open, Australian Open and French Open victories with Czech partner Lucie Safarova, who went to the court when she heard about the injury.
Mattek-Sands has also received support from the tennis world. Nine-time Wimbledon singles champion Martina Navratilova wrote on social media: "Am just devastated about Bethanie Mattek-Sands and her injury - we are all behind you, hoping for the best - lots of love, m."
American Wimbledon singles champion Chris Evert added: "Going over the day and remembering @BMATTEK [Bethanie Mattek-Sands] in my thoughts and prayers and wishing this very special lady a speedy recovery..."
Andrew Castle, BBC Sport tennis commentator:
The club monitor the moisture levels of the grass at all times but when it's hot, it wilts and dies and then can become slippery.
I spoke to the Centre Court groundsman who told me his plan. This morning, he put the roof over and soaked it to give it a drink and a rest. We all need that every now and again.
The trouble is the players aren't used to it as they don't play on it for much of the year - they are mostly on clay or hard courts.
Hopefully they understand it's a natural surface and it does show natural signs of wear and tear.
David Cameron has said that he does not expect an agreement to be reached on his EU reform aims at December's summit of European leaders.
[NEXT_CONCEPT]
Bethanie Mattek-Sands is having more scans on a serious knee injury she picked up during her second-round Wimbledon singles match on Thursday.