Uncertainty over the UK’s future in the EU is destabilising Europe, the president of the European council has warned as he said that no agreement had been reached on controversial demands by David Cameron to limit access to in-work benefits.

In a letter to the EU’s 28 leaders, Donald Tusk says the prime minister has tabled some difficult demands that cannot be agreed in time for the European council next week.

Tusk cites the prime minister’s proposal to ban EU migrants from claiming in-work benefits for four years as the most troublesome UK demand.

But the Polish politician, who says he is acting as an honest broker in the negotiations, instead suggests he is confident that a concrete proposal can be adopted at the next summit in February.

“Our goal is to find solutions that will meet the expectations of the British prime minister, while cementing the foundations on which the EU is based,” he writes. “Uncertainty about the future of the UK in the European Union is a destabilising factor. That is why we must find a way to answer the British concerns as quickly as possible.”

In a point-by-point response to Cameron’s four demands, Tusk describes the prime minister’s proposal for a four-year ban on EU migrants claiming in-work benefits as the most difficult. He writes: “While we see good prospects for agreeing on ways to fight abuses and possibly on some reforms related to the export of child benefits, there is presently no consensus on the request that people coming to Britain from the EU must live there and contribute for four years before they qualify for in-work benefits or social housing.

“This is certainly an issue where we need to hear more from the British prime minister and an open debate among ourselves before proceeding further.”

On the prime minister’s other demands, Tusk writes:

There is already agreement that the EU’s commitment to create an “ever closer union of the peoples of Europe” allows for what he calls “various paths of integration”. The prime minister wants Britain to be given a legally binding commitment, embedded in EU treaty change, that this does not apply to the UK.

EU leaders are open to the idea of providing guarantees to the UK and to other EU members outside the eurozone that the “euro-ins” will not impose their will on the “euro-outs”.

There is strong agreement on the need to promote economic competitiveness.

Tusk says that good progress is being made but EU leaders will need to have a “substantial political debate” at next week’s summit. This will allow for an agreement on concrete proposals in February.

In his letter, Tusks pleads with all sides to work constructively. He writes: “All involved must take their part of responsibility. I will act as an honest broker but all member states and the institutions must show readiness for compromise for this process to succeed.”