David Cameron's EU renegotiation has been dealt a fresh blow after the president of the European Council branded some of his demands "unacceptable".
Donald Tusk dismissed the prospect of the Prime Minister getting his counterparts to sign up to all his proposals - the most contentious of which is a four-year ban on in-work benefits for migrants.
The comments, ahead of a potentially stormy Brussels summit in which Mr Cameron will make a personal appeal to the other 27 leaders to grant concessions to Britain, come after German Chancellor Angela Merkel warned she would not allow any changes that "discriminated" against EU nationals.
European Commission president Jean-Claude Juncker has said any deal for the UK must be "fair" on other countries, while European Parliament chief Martin Schulz said MEPs have enormous scepticism about the plan.
The Prime Minister is under intense domestic pressure to secure substantial concessions for Britain ahead of the referendum on EU membership promised by the end of 2017.
Officials say he will make a "direct" and "constructive" appeal to counterparts over dinner, insisting they must respond to UK concerns about a lack of control over immigration, the effects of closer eurozone integration, the single market and a lack of competitiveness.
Mr Cameron is again due to raise the idea of the four-year ban - a Conservative Party manifesto pledge at the general election.
He has indicated he would be willing to consider alternative ways of cutting immigration "pull factors", although British officials say no other ideas have yet been tabled.
Speaking to reporters at the Council building in Brussels, Mr Tusk said parts of the British demands seem "unacceptable".
He said: "The consultations I have led with all member states shows goodwill of all the parties involved, but it doesn't change the fact that some parts of the British proposal seem unacceptable.
"However, if Prime Minister Cameron persuades leaders tonight that we can work together to find solutions regarding all four baskets, then we will have a real chance to strike a deal in February."
Asked at a separate press conference whether there is any chance of getting the ban, Mr Juncker said: "We will enter the concrete and vital phase of negotiations with our British colleagues, the Commission is ready to look for other options to the single one proposed by the British Prime Minister and I am quite convinced that we will find an answer to that highly complicated question."
Mr Juncker told reporters that any concessions offered to the UK must be fair to the wider EU as well.
"We want a fair deal with Britain and this fair deal with Britain has to be a fair deal with the other 27 too, so we are open-minded," he said.
"We are engaging in this dialogue which will be a negotiation with Britain in an open-minded way.
"I don't want the British to leave and I don't want to blame the British - they have their points, we have our points and as reasonable people we will find a way out of the complicated situation we are in."
A number of eastern European states have already indicated they would veto Mr Cameron's plan as discriminatory and breaching the EU principle of free movement of labour.
The leaders are not expected to reach a final deal at this two-day summit, but Mr Cameron and Mr Tusk are hoping to "pave the way" for an agreement at the next gathering in February.
Former European Council president Herman Van Rompuy suggested Mr Cameron would struggle to secure any changes that require reform of the EU treaties.
"They know there is a risk and they want to keep Britain on board but language or good speech will not do the job; it is about the substance," he told BBC Radio 4's Today.
"We can look for solutions, within certain constraints. If you want to change the treaties then we have a problem."
Mr Van Rompuy said Mrs Merkel is open to compromise but "a lot of creativity" will be required.