A deal on changes to the UK's relationship with Brussels will be "very difficult" to achieve before Christmas but the president of the European Council said he would help David Cameron if he gambled on pressing for an agreement at a meeting later this month.
Donald Tusk, who will chair the summit on December 17, said both he and the Prime Minister agreed that it would be better to do a deal this month rather than wait until the next meeting in February, but indicated it would be a risk for the Prime Minister to push for a rapid decision.
Mr Cameron 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 will allow him to make the case for staying in and give him more flexibility in choosing the date for the public vote.
But if European leaders fail to agree on an acceptable package of reforms, the Prime Minister has insisted he will not rule out recommending a vote to quit the EU.
Mr Tusk told the Guardian that both he and Mr Cameron thought December would be better than February for a decision.
"If he is ready to take this risk, I will be helpful. But then, it would be his risk," Mr Tusk said. "If Cameron is sure December is better for him as the organiser of this referendum, I will be helpful and I am ready to convince our officials."
The council president said he did not have a final draft of a settlement to put on the agenda but "it is quite possible to prepare this document for December".
"If David Cameron, together with me ... if we decide that it is safe enough to shape it up, then we will take the risk.
"The first political priority is obviously to help Cameron to win the referendum. It means that I really cooperate very closely with David Cameron also when it comes to the question of the timing. I have no doubt that the first goal must be to keep the UK in the EU."
But with officials cautious about a rush to do a deal, Mr Tusk warned that it could be safer to wait until the February meeting.
A deal this month is "still possible but very, very difficult", he warned.
"My intention is to avoid the risk that, you know, we will show the final proposal, but with some doubts or protests from the member states. It means that the risk of failure is then very, very clear. Maybe I am too cautious, but my first advice was to continue our work and maybe February is more realistic and safer. But I understand the arguments from the other side. I mean, of course, the Brits."
Potential stumbling blocks include Mr Cameron's proposal to block EU migrants from claiming in-work benefits for four years and an agreement to safeguard the interests of non-eurozone countries.
Mr Tusk suggested it was possible that EU leaders could agree to a political statement in favour of reforms, leaving the complex legal amendments and changes to EU treaties until after the referendum.
"If I understand well Cameron's intentions, he needs honest and responsible declarations from all member states that they're ready to change some rules according to what he expected. We're not talking about legal actions and procedures. It's impossible to prepare the whole legal process before the referendum.
"Now everything is in our hands: the British Government and my office here. For sure if the risk of possible Brexit is more visible, more real, it will be at the top of our heads in all capitals."