David Cameron is heading to Paris to try to get President Francois Hollande on board for his renegotiation of Britain's EU membership, as Downing Street confirmed that some details of his reform package still need to be "pinned down".
France has raised concern that proposed protections for non-eurozone states could amount to special treatment for the City of London.
But Downing Street declined to say whether the hastily-arranged meeting on Monday evening was sparked by concerns that Paris could block agreement on reforms at a crunch Brussels summit at the end of this week.
Failure to get unanimous support from all the other 27 national leaders could delay the promised in/out referendum until after the summer. Foreign Secretary Philip Hammond, who was meeting EU counterparts in Brussels, said there were still "very big issues" on the table.
Mr Cameron's official spokeswoman said: "What we have seen from the French throughout, and from President Hollande personally, is the French wanting to work with us so we can both agree on the solutions that the UK needs."
European Council president Donald Tusk is undertaking a whirlwind tour of EU capitals including Berlin, Paris and Athens to sell the package of reforms which he has drafted in response to Mr Cameron's demand for change - and he updated the PM on progress in a 10-minute phone call on Monday morning.
Asked whether Mr Cameron was confident of securing a deal in the two-day summit, starting on Thursday, the spokeswoman replied: "We have made progress but there are details to be pinned down. Nothing is agreed until everything is agreed."
Mr Cameron will meet the president of the European Parliament Martin Schulz in Brussels on Tuesday morning, along with the three MEPs who have been appointed "sherpas" for the renegotiations - Guy Verhofstadt, Roberto Gualtieri and Elmar Brok. He will then have separate meetings with leaders of the Socialist and EPP groupings in the European Parliament, Gianni Pitella and Manfred Weber.
But he will not meet leaders of other groupings, including Ukip's Nigel Farage, who chairs the European Freedom and Direct Democracy group, as had been expected.
Downing Street confirmed that elements of the reform package, involving restrictions on welfare payments for migrant workers and cuts in child benefit for offspring living overseas, will require legislation in the European Parliament even if they are approved by national leaders in the European Council.
Asked whether this could put MEPs in a position to block the changes after the referendum has taken place, the PM's spokeswoman said: "This would be a legally-binding document under international law, entered into by the 28 leaders of member states. There are a number of decisions subsequently which would have to be delivered by secondary EU law by co-decision with the European Parliament."
She added: "We will be looking for all countries in the European Parliament to deliver on that, following the decision of the British people."
Meanwhile, HSBC warned that it could move 1,000 staff to Paris if the UK votes to leave the EU in the upcoming referendum.
Following a lengthy review, the group announced it was keeping its HQ in London, and chief executive Stuart Gulliver said that the result of the referendum would not impact on this decision.
But he added: "If the UK leaves the EU it could have a significant impact on our non-ring-fenced bank - our trading room, corporate banking and investment banking ... We have 5,000 people in global banking and markets in London and I could imagine that around 20% of those would move to Paris."
Responding to the comments, the Prime Minister's official spokeswoman said: "The PM has talked many times about the issues and choices the British people will have to weigh up when they make a decision on whether the UK should remain or leave, that there are economic risks.
"But he has also talked about the confidence we should have in our country and our ability to succeed."
Securing a deal this week on Mr Tusk's package is all-but-essential if Mr Cameron is to press ahead with a referendum on June 23, widely thought to be his preferred date.
As well as French concerns over financial proposals, he is also facing objections from eastern Europe to plans to scale back welfare payments to migrant workers, with countries such as Poland thought to be holding out for any restrictions to be applicable to the UK only.