David Cameron has insisted he is in "no hurry" to secure a deal on a package of reforms to Britain's EU membership at the upcoming Brussels summit in February.
A deal at the European Council of February 18-19 is widely seen as essential if Mr Cameron is to stage his promised in/out referendum before the summer.
But speaking to the World Economic Forum in Davos, Switzerland, Mr Cameron declared he was ready to be "patient" in order to get the right result.
Securing a deal next month was "achievable (and) doable", he said. But he added: "We are certainly not there yet."
And prospects for a swift agreement were played down by French Prime Minister Manuel Valls, who said the negotiations had "only just begun" and warned that a deal "at any price" would not be acceptable.
However, Mr Cameron issued a plea for businesses and charities not to delay making the case for continued EU membership until after the negotiations are complete.
Although he has told Cabinet colleagues that they must hold their silence on Europe until a package is agreed, he made clear it was not necessary for business leaders and non-governmental organisations (NGOs) to do the same.
"I hope that business and NGOs and other organisations won't hold back," said the Prime Minister.
"And I would say don't hold back right now, even though the question isn't settled. I think that if business backs my reforms, if you want to see the competitive Europe, if you want to see the flexible Europe, if you want to see a Europe where you can be in the eurozone and win or out of the eurozone and win, I would argue 'Get out there and support those things'.
"I think it's important that with this, which is such a massively important generational question for Britain and for Europe, the sooner you can start to look at your own businesses and come up with the examples and the ideas about the benefits and the problems that there are with Europe, the more you are able to help to explain and set the context for this vitally important question for Britain and for Europe."
Making clear he was ready to delay agreement on a reform package until after February, Mr Cameron said: "I very much hope that we can, with the goodwill that is clearly there, reach an agreement at the February European Council. I would like that. I want to confront this issue, I want to deal with it, I want to put that question to the British people in a referendum and go out and campaign to keep Britain in a reformed European Union.
"If there's a good deal on the table, I will take it and that's what will happen. But I do want to be very clear - if there isn't the right deal, I'm not in a hurry. I can hold my referendum at any time up until the end of 2017, and it is much more important to get this right than to rush it.
"But of course I think it would be good for Europe and good for Britain if we demonstrated that we can turn the goodwill that there is into the actions that are necessary to put this question beyond doubt and get the answer from the British people."
Speaking to an audience of international political and corporate leaders at the annual gathering in the Swiss ski resort, Mr Cameron said his aim was "to secure the future of Britain in a reformed EU", which he said would be good both for Britain and for Europe.
Britain's demands for change on the four issues of migration, sovereignty, competitiveness and protection for non-euro states were "not outrageous asks", but offered "a huge prize", he said.
Success in the renegotiation could deliver the UK "the best of both worlds" by allowing it to be within the single market while retaining control of its borders and remaining outside the single currency and the ever-closer union sought by some other member states.
"I think that is a prize worth fighting for, it's a prize worth negotiating for, if necessary it's a prize that we will have to be patient in order to achieve, but it's a prize I'm determined to deliver in this, my second term as Prime Minister," said Mr Cameron.