A deal on reforms Britain has demanded to stay in the European Union will be put in writing by early February for EU leaders to discuss later that month, a European diplomat said Wednesday.

British Prime Minister David Cameron has said he is reasonably confident of an accord for the changes in four key areas, ahead of a referendum on a possible "Brexit" from the bloc, expected in mid-2016.

"I think that a concrete draft could be put on the table in early February," the European diplomat said on condition of anonymity. "Between now and February, the process continues."

Officials from Britain, the other 27 EU governments, and the European Commission were having "intense discussions" in a bid to hammer out an agreement, the diplomat added.

EU leaders will hold a summit in Brussels on February 18 and 19 at which they have said they hope to reach a deal.

The referendum must be held by the end of 2017 but Cameron on Sunday gave his clearest hint yet that it could come later this year, telling the BBC: "I would like to see a deal in February and a referendum that will follow."

Cameron said his most controversial proposal -- a four-year ban on top-up benefits for migrants -- was "still on the table" but he could agree to an "equally powerful" plan to tackle so-called benefit tourism.

His other demands -- for the European Union to give Britain safeguards against more political integration, to protect countries that do not use the euro currency and to boost economic competitiveness -- face much less opposition.

Belgian Foreign Minister Didier Reynders warned however on Wednesday that Cameron could not expect an "a la carte" form of EU membership for Britain.

"There are red lines for us," Reynders told journalists in Brussels. "We will not go backwards unless everyone (all 28 EU states) decides that we will generally transfer some powers back to individul states."

Reynders also voiced caution over Cameron's demands for protections for non-euro nations.

"Cameron is afraid that the steps we take in the eurozone could harm the City, for London's position... but it's a bit strange to worry about the euro when he is always telling us he wants no part in it. He has to figure out what he wants!," the Belgian said.

"Nor can we have countries that are not members of the euro and do not want to be blocking procedures in the eurozone.

"So we have to find formulas, and there are formulas" for a deal, he added.