David Cameron has been given a fresh warning about his European Union renegotiations after one of the most senior figures in Brussels indicated the Prime Minister's key demand is unacceptable.
The Prime Minister is braced for a showdown with EU counterparts at a crucial summit in Brussels, with opponents lining up against his call for a four-year ban on new arrivals in the UK claiming in-work benefits.
European Commission president Jean-Claude Juncker said a deal for Britain must also be fair to the other 27 members, while European Parliament chief Martin Schulz said MEPs have enormous scepticism about the plan.
"My and my colleagues' doubt is that a four years' delay is acceptable," Mr Schulz said at a Brussels press conference ahead of the talks.
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 the other 27 leaders 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.
But challenged on whether there is any chance of getting the ban, Mr Juncker, the EU's most senior official, indicated they would look at "other options" instead.
He 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.
German Chancellor Angela Merkel also warned she would not allow "discrimination" against migrants.
Addressing the Bundestag, Mrs Merkel described Britain as a "natural ally" and stressed the "enormous importance" of avoiding so-called "Brexit". But she made clear that Berlin is not prepared to give way on all of Mr Cameron's demands.
"We don't want to, and we won't, call into question the core principles of European integration," she said. "These include in particular the principle of free movement and the principle of non-discrimination between European citizens."
Mr Cameron has signalled he is willing to consider other proposals that would help reduce migration. But a senior UK official said although there are "murmurings" of alternatives - such as an "emergency brake" mechanism for when inflows increases sharply - the four-year benefit ban is as yet the only option formally on the table.
The leaders are not expected to reach a final deal at this two-day summit, but Mr Cameron and European Council president Donald 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.
"We need now a lot of creativity to tackle the problem posed by Britain but at the same time not crossing those red lines of non-discrimination," he said.
"There are possibilities. I am not entering into details but within the constraints, and given the problem posed by Britain, a compromise is possible. That is also the opinion of the German chancellor."
Ukip leader Nigel Farage told Today: "We really should be discussing a new relationship. However we are not.
"All the big questions - the Foreign Secretary talking about fundamental treaty change, Mr Cameron talking about our ability to control open borders - none of this will even be discussed tonight.
"The only substantive point the Prime Minister has asked for is for us to be able to limit migrant benefits for up to four years, and he is not even going to get that."
Meanwhile, Labour leader Jeremy Corbyn is also in Brussels, attending a meeting of the Party of European Socialists (PES) parliamentary bloc.
He will tell the meeting: "The Prime Minister has botched his negotiations with European leaders. He has tried to bludgeon them into accepting flawed and phoney reforms, which will not address the real problems of the European Union - and failed.
"They have called David Cameron's bluff, and he knows it. Labour backs Britain's continued membership of the EU as the best framework for trade and co-operation in a 21st-century Europe. And we will campaign for Britain to stay when the referendum is finally held.
"But people across Britain and Europe know that the EU needs to change if it's going to work better for the majority of its people, not just its banks and corporations."