David Cameron’s quest to secure a new deal for Britain in Europe by reopening the EU treaties is virtually “mission impossible”, according to the key EU politician who will run the negotiations. But Donald Tusk, the president of the European council chairing the summits that will decide the answer to the British question, told the Guardian he will offer limited help to the prime minister in renegotiating the terms of the UK’s EU membership.

A little more than three months before Cameron, should he remain prime minister, is expected to deliver his wish list to Brussels on what Britain needs if he is to win a referendum keeping the UK in the EU, Tusk said he would be hard-pressed to engineer a consensus among the other 27 EU countries that would satisfy the prime minister.

“I am ready to help Cameron,” Tusk said in an interview with the Guardian and five other European newspapers. “I have no doubt that we have to help in a limited and rational framework. We have to help David Cameron because he is obviously pro-European. I am sceptical when it comes to changing the treaty.”

The UK government has repeatedly pushed for a re-opening of the EU’s Lisbon treaty to nail down the changes it wants, arguing that the pact has to be rewritten to underpin greater political integration in the crisis-ridden eurozone. Tusk said there were minimal chances of this happening.

“My intuition is that treaty change is close to mission impossible today because it’s not only about rationality, about good argument,” he said. “We need unanimity between 28 member states, in the European parliament, in 28 national parliaments in the process of ratification. To say that it is a Pandora’s Box is too little.”

Tusk’s comments were the first delivered publicly on the British issue since he took office as European council president in December. A former Polish prime minister, he promptly listed the British issue as one of the three biggest items in his in-tray when he took the job.

Cameron’s attacks on free labour movement within the EU and his questioning of immigration from elsewhere in the union, particularly eastern Europe, make the British quandary tricky for Tusk as a former leader of Poland, the biggest source of Europeans migrating to the UK.

“It’s not easy for me because of some expectations close to the immigration problems, which means also the huge Polish minority in Great Britain,” Tusk said.

Cameron intends to call a referendum in 2017 on whether Britain should stay in the EU, hoping first to have negotiated what he sees as a better deal for Britain that he can put to the vote. But the prime minister has been told privately that the Europeans are in no mind to reopen the Lisbon treaty and that he is unlikely to enjoy the support of the EU’s most powerful figure – Germany’s chancellor, Angela Merkel.

Ahead of the UK’s general election in May, say senior officials and diplomats in Brussels, the EU and Britain are in a “phony war” phase, waiting to see who will be prime minister and expecting Cameron, if he remains in office, to come to an EU summit in June and reveal his hand.

“First of all we need more precise concrete [details] about British demands. When we have more information about details, about legal problems, then we can decide if discussion about treaty change is at all necessary,” said Tusk. “For me the problem of possible treaty change is a practical question. It’s not only about the arguments of Cameron. I can agree or disagree with some of them. But at the end of the day I would like to ask him ‘what is your concrete proposal?’ and then I can consult with the other member states.”

Given five years of single currency turmoil that rocked the EU to its foundations, Tusk said that reopening the Lisbon treaty to advance eurozone integration was a much more important question for the EU, but that even on this there was no stomach for such a move.

“The first reaction of member states about a possible process of treaty change in this context was in fact univocal, that they don’t want today any discussions about treaty change. We need, also for the United Kingdom, a good solution for Cameron and Great Britain under existing law,” he said. “We need the United Kingdom in Europe. For me there’s no more obvious political thesis than this one. And I feel, but it’s not my role to decide about it, but I feel that the United Kingdom needs Europe.”