David Cameron’s hopes of securing a new basis before Christmas for keeping Britain in the European Union are fading, according to his closest ally in the EU, amid strong signs that it will be next February before the prime minister is able to finalise new membership terms to be put to a referendum.

“I am not sure we will get to a conclusion in December. That has to be seen,” said Mark Rutte, the prime minister of the Netherlands. “At least we should be able to make a very serious start.”

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The British have been assiduously courting the other 27 EU governments since Cameron for the first time delivered a written list of UK demands to the Europeans last month. The aim was that a summit on 17-18 December would wrap up the negotiation, enabling the prime minister to push for a referendum date as early as next summer. That timetable is slipping as the urgent issues of mass migration and post-Paris counterterrorism dislodge the “British question” from the agenda of EU summitry.

Senior EU diplomats credit the prime minister and his aides with pursuing a much more positive and conciliatory negotiating strategy in recent weeks, but the French in particular are highly critical of most of the British demands, while the British aim of curbing in-work benefits for mainly low-paid east Europeans in Britain is generally seen as a non-starter.

Briefing journalists at his office in The Hague, Rutte made it plain that he was sympathetic to much of the British case, but said there were two big problems – freezing benefits for east Europeans and successfully negotiating a new deal that would protect Britain from being outvoted by the 19 countries that use the euro.

“When I take a look at what David Cameron has proposed, I think some of them are doable and some of them are more difficult, and it will take some time to really go into detail and to discuss that,” Rutte said. “I can understand why the UK is worried about a sort of automatic qualified majority vote for the eurozone and what that could mean for the City [of London]. I do understand that he wants some reassurance on how to deal with that. I am not sure exactly how we are going to do that without hurting some of the other interests.”

Cameron is proposing to freeze tax credits and housing benefits for four years for EU citizens working in the UK.

Two ambassadors to the EU have separately told the Guardian this was the biggest sticking point in the negotiations. There is speculation that Cameron is seeking a face-saving way of climbing down on a commitment that was emphasised in the Tory election manifesto. “Politically and legally, the most sensitive issue is the four years and welfare curbs,” one of the ambassadors said.

“Social security is the really tricky issue from every point of view,” said the other.

On the proposed welfare freeze, Rutte said: “We could also find common ground: not to do exactly what [Cameron] has asked for, [but] something that would deliver what he needs within what is acceptable for eastern European countries. Where exactly that landing spot is, I don’t know.”

Rutte and the Dutch generally are keener than anyone in the EU to keep Britain in, and the Dutch leader counts Cameron as a personal friend. The Dutch also take over the rotating presidency of the EU from January for the crucial six months when the outcome of the negotiations will be settled.

“We very much want to help, but we have to find some middle ground on these issues,” said Rutte. “For us in the Netherlands, and not because [Cameron] is a personal friend – yes, that is also nice – but in the end it is about interests. And for the Netherlands, the membership of the UK is vital because it is one of the few countries that is market- and growth-orientated.

“He is a good personal friend. On this particular issue of negotiating a deal, I want it to be successful. But I help nobody if I’m his friend or somebody else’s friend, particularly if this is taking place during our presidency. Then we have to be the honest broker and impartial judge.”

Donald Tusk, the president of the European council, who chairs EU summits, is seeking to craft a settlement acceptable to all and his aides have been conducting a series of “confessionals” on the British demands with the other EU governments in recent weeks.

The French are said to be cutting up rough on several fronts, stating that it will be almost impossible for Cameron to secure the “permanent, legally binding, irreversible” changes the UK is demanding without rewriting the Lisbon treaty, which cannot be done.

The British demand that the EU be declared a “multi-currency union”, putting the pound on a par with the euro, is also rejected by France. And while the French agree that the welfare issue is the biggest immediate problem, they are more concerned about the longer-term impact of a binding deal that ties the hands of the eurozone in a concession to Britain.

“That’s the issue that is most systemic and that we will be paying most attention to,” said a senior source. “It will be very difficult to accept.”