David Cameron is braced for a showdown with EU counterparts over his renegotiation demands after German chancellor Angela Merkel warned she would not allow "discrimination" against migrants.
The Prime Minister is under intense domestic pressure to secure substantial concessions for Britain as he heads for a crucial summit in Brussels.
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 a four-year ban on new arrivals to the country claiming in-work benefits - a Conservative Party manifesto pledge at the general election.
But a number of eastern European states have already indicated they would veto such a plan as discriminatory and breaching the EU principle of free movement of labour.
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 was not prepared to give way on all 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 were "murmurings" of alternatives - such as an "emergency brake" mechanism for when inflows increased sharply - the four-year benefit ban was 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.
A number of Tory backbenchers have already dismissed the renegotiation as lacking ambition, saying it would make little difference even if the PM achieved all his goals.
More evidence of the tightrope act Mr Cameron is having to perform emerged yesterday when his predecessor Sir John Major cautioned against "flirting" with leaving the EU.
Sir John, whose own leadership was undermined by bitter internal rows over Europe, said: "I am sceptical of a great deal of European Union policy. But flirting with leaving, at a moment when the whole world is coming together, seems to me to be very dangerous and against our long-term interests."
Asked about the comments at Prime Minister's Questions, Mr Cameron said: "What I will be doing is getting the best deal for Britain.
"This government was the first to cut the EU budget, it was the first to veto a treaty, the first to bring back substantial powers to Britain. We have got a great record on Europe and we will bring back a good deal for the British people."
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.
"Cameron's timid and lop-sided demands, choreographed for the cameras, won't achieve that."