David Cameron is to hold talks with European Commission President Jean-Claude Juncker about his demands for EU reform, amid indications that negotiations are proceeding more slowly than hoped.
Mr Juncker said on Wednesday that the talks, which kicked off in June, are not making "huge progress" - though he also acknowledged that "I can't say that nothing has been achieved".
In an apparent sign of frustration with the UK, the Commission president told the European Parliament that "it takes two to tango" and that "our British friends have to dance".
Reports have suggested that it may be March before leaders of the 28 EU member-states meet to finalise any package of reforms.
But Downing Street has played down any suggestion that its preferred timetable is slipping, insisting that Mr Cameron has made clear all along that the process will be driven by progress in the negotiations and not by arbitrary deadlines.
The Prime Minister has promised an in/out referendum on Britain's EU membership by the end of 2017, though most observers believe he would rather have it out of the way by the end of next year to avoid clashing with German and French elections. Europe minister David Lidington told MPs earlier this week that a poll in the second half of 2017, when the UK holds the EU's rotating presidency, would not be an "optimum solution".
Mr Cameron aims to use his working lunch with Mr Juncker ahead of a summit of the European Council in Brussels as an opportunity to maintain momentum in the renegotiation process, said British officials.
The talks, which have so far involved technical discussions between officials of the Government and EU institutions, are expected to broaden out to include leaders of other European states over the next two months before a "substantive discussion" at a Council meeting in December.
Today's summit will receive an update from Council President Donald Tusk, but is unlikely to feature any debate over Britain's demands - including changes to welfare rules for migrant workers, the lifting of member-states' commitment to "ever-closer union", measures to protect non-euro states and greater powers for national parliaments.
The bulk of the meeting - which is expected to be the target of protests by French farmers - will instead be taken up by the migrant crisis, with discussions also touching on the situation in Ukraine, eurozone economic and monetary union and the upcoming climate change conference in Paris.
Mr Cameron will meet European Parliament President Martin Schulz to give his response to his invitation for the PM to address MEPs on his referendum plans.
Mr Juncker sparked ferocious debate in Brussels when his unclear diction left observers unable to agree whether he had told the Parliament that Britain needed the EU.
His spokeswoman insisted he said "personally I do think that Britain needs the European Union", but Ukip leader Nigel Farage insisted that he had heard Mr Juncker say the word "don't" rather than "do", and offered to buy him a bottle of champagne in celebration of the moment.
Mr Juncker said "the Commission wants a fair deal with Britain and we are working in that direction".
But he added: "I can't say that huge progress has been achieved. I can't say that nothing has been achieved.
"It needs two to tango so we have to dance and our British friends have to dance. I am 150% in favour having Britain as a constructive member state of the European Union."
Responding to Mr Juncker's comments, the PM's spokeswoman said: "We are seeking to renegotiate the UK's relationship with the European Union. That is going to take time and it's going to take patience and what matters is that we get the right reforms to address the concerns of the British people."
Mr Cameron has welcomed European Commission plans to push ahead with trade deals which he claims could result in a £20 billion boost to the UK economy.
The new trade strategy will prioritise concluding agreements including the controversial Transatlantic Trade and Investment Partnership (TTIP) with the USA, an EU-Japan free trade deal and work on an EU-China investment agreement.
It also opens the door to new negotiations in the Asia-Pacific area, including with Australia, New Zealand, the Philippines and Indonesia.
Following anti-TTIP protests, trade commissioner Cecilia Malmstrom insisted the strategy would be "more responsible" and reflect concerns about the impact of deals on public services and human rights.
The Prime Minister said: "This is firmly in Britain's interests and it's proof of how we can persuade the European Commission to focus on actions that will create growth and jobs here at home."