David Cameron is pressing 27 other European heads of government to accept his terms for deciding Britain’s future in the EU at a Brussels summit in two weeks’ time, the summit chairman said.

Donald Tusk, the president of the European council, who will chair the crucial summit on 17 December, signalled that Cameron was in a hurry to resolve the vexed issues on Britain’s future in the EU before Christmas.

Tusk said he agreed personally with Cameron on the drive to conclude the UK-EU negotiations before Christmas and would assist the prime minister. But he also voiced reservations about the highly fraught negotiations since central and complex issues in the British list of demands remained unresolved.

“In fact, his personal opinion and mine were and are that December is better than February,” Tusk told the Guardian, referring to the next two EU summits. “If he is ready to take this risk, I will be helpful. But then, it would be his risk … 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.”

Tusk has days to decide whether to put a final UK ultimatum on the agenda for the pre-Christmas EU summit in Brussels on 17-18 December, with the aspiration of determining the matter at that meeting. His staff are in daily contact with Downing Street over whether to submit a text to the other European leaders. If agreed, the document would form the basis for Cameron’s in/out referendum, which could take place by next summer. It has to be held by the end of 2017.

Top officials on both sides are worried about rushing the issue, preferring to leave an attempted resolution until the following EU summit in February. However, Tusk said he and Cameron believed a quick deal was feasible politically, but that there were major legal problems with UK demands for a much looser relationship with the rest of the EU. Cameron discussed the issue with Tusk last Sunday. Witnesses to the conversation said the prime minister was “forceful” about pressing for a quick decision in two weeks.

Cameron and Tusk are to talk by telephone on Thursday morning when a final decision on the timing is likely to be taken. While there is no formal deadline for a decision, the other governments would need at least 10 days to study the text of a proposed settlement. This would come from London and then be pored over by Tusk’s office. The EU president has not yet received the document from the UK.

“I do not have the final draft,” Tusk said. “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 Tusk also said that his instinct was to give the UK negotiations more time until February, although he “understood [Cameron’s argument] that in fact maybe we have to play against time”.

Eurocrats and Whitehall mandarins are much more “sceptical and cautious” about rushing a potential deal, Tusk said, a view echoed by senior officials involved on both sides in Brussels.

But they all agree that Cameron could bypass the officials doing the spadework on a proposed settlement, that the prime minister is prepared to gamble on taking the issue to the brink in a fortnight by forcing it on to the government leaders’ agenda, which is already packed with the counter-terrorism and mass migration crises.

Tusk stressed repeatedly that he was determined to forge a deal that could win Cameron’s referendum and keep Britain in the EU, but agonised over the details given the existing major sticking points.

The two biggest issues concern Cameron’s aim of scrapping for four years in-work benefits paid to EU immigrants in Britain and crafting a new legally watertight regime making it impossible for the 19 countries using the euro single currency to outvote non-euro countries such as Britain.

George Osborne, the UK chancellor, has demanded that the EU be defined as a “multi-currency” union, putting the pound on a par with the euro. This has been rejected by everyone else, senior sources said.

There is broad agreement on a new “emergency brake” mechanism allowing the British to delay decisions by European finance ministers if they are deemed to damage UK interests or threaten financial instability in the UK. Britain is insisting that the mechanism entails “delay and escalation”, meaning that finance ministers would shelve a decision that would then be tackled by national leaders at summit level.

The German finance minister, Wolfgang Schäuble, is known to oppose this. Several other governments are worried that the “delay and escalation” measures would, in effect, turn into a UK veto over financial decision-taking, although Britain insists it does not want a veto. Also unresolved is whether the emergency brake would privilege only Britain or whether other non-euro countries could trigger it.

The other main sticking point concerns different welfare treatment of Britons and EU immigrants in the UK. Cameron continues to insist on the four-year benefits freeze for immigrants, stipulated in the Tories’ last election manifesto. The eastern European governments are digging in their heels while being supportive of the rest of the UK arguments.

“For some reasons the eastern part of Europe is more ready to accept Cameron’s demands and requests. I am quite sure. More for geopolitical reasons,” said Tusk in the wide-ranging interview with the Guardian and five other European newspapers. “But we have more problems with lawyers. This is a legal problem – the four years issue of the benefits for people in work.

“The legal aspect is the most difficult thing, not the lack of will or of compromise among member states. I’m really impressed about the willingness from the most affected countries to help Cameron. But it’s very difficult because of treaty rules. Now we’re working on some legal possibilities, but of course we need some compromise when it comes to substance.”

Cameron put his demands in writing to Tusk last month for the first time. This was followed by eurocrats and Tusk’s staff holding 27 individual sessions with the other EU governments. These so-called “confessionals” were concluded on Monday last week. They were also “shadowed” by Cameron’s aide, Tom Scolar, and the UK ambassador in Brussels, Ivan Rogers, who held their own parallel sessions with each of the 27 governments.

A deal this month, Tusk said, is “still possible but very, very difficult. 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.”

Tusk said a pre-Christmas breakthrough could take the form of EU leaders issuing a “political declaration”, parking the legalistic dilemmas for the future.

“What we need in December – also in February if we have to postpone it – is rather a political declaration because it’s impossible to change the [EU] treaty before 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.”