Britain's push to win backing from its European partners for its wish list of EU reforms will go "right to the wire" at a summit this week, Foreign Secretary Philip Hammond says.
"There isn't a deal yet, there is a working draft, there are lots of moving parts and we have got a negotiation that will run through this week, and I have no doubt will run right to the wire," he told BBC television on Sunday.
He said progress was needed to nail down key demands in the areas of competitiveness, the relationship between countries in the bloc that use the euro and those that do not, national sovereignty and access to welfare benefits.
British and EU negotiators have already broadly agreed much of a reform package, but tricky political issues, notably on migration, are still outstanding.
Prime Minister David Cameron is hoping to return from a summit of European leaders on Thursday and Friday in Brussels with a package of reforms that he can take to the British people in a referendum on whether to remain in the EU.
"Our European partners understand that we have to have a robust deal in each of those areas if the British people are to vote to remain inside the European Union," Hammond told the Andrew Marr show.
The campaign to remain in the bloc stepped up a gear on Sunday, when the boss of airline easyJet said Britain's membership of the EU was the reason that the cost of flights had plummeted, while the range of destinations had soared.
"Whatever way to look at it, the EU has brought huge benefits for UK travellers and businesses," Carolyn McCall wrote in the Sunday Times.
"Staying in the EU will ensure that they, and all of us, continue to receive them."
Campaigners to leave, however, repeated claims that EU supporters were running a fear campaign to scare people into voting to stay.
"Those that wish to remain in the EU should make the positive case for the supranational European project rather than frightening people," former defence minister Liam Fox told the newspaper.