David Cameron's drive to reform the EU ahead of a mooted in-out referendum was given a major boost by the new president of the European Council tonight.
Donald Tusk, who will give up his job as Polish Prime Minister to take the role, singled out the UK's agenda as an area where he wants to make progress.
He said he "could not imagine" the EU without Britain as a member - insisting it would be possible to "reach an agreement" on issues such as freedom of movement of labour.
At a press conference after his appointment was finalised by fellow European leaders in Brussels, Mr Tusk said he wanted to "emphasise that the EU, and me personally, will take on the concerns voiced by the UK".
According to the official translation from Polish, he said: "I am talking about the UK because I am sure that the future of the EU is not about making it smaller, about contraction.
"No reasonable person can imagine the EU without the UK. I cannot imagine it myself.
"I have talked about it with David Cameron. He put forward many proposals for reforms and I am sure with a reasonable framework of politicians we can reach an agreement.
"We can strive to eliminate various barriers - freedom of movement of workers, for example."
Pawel Swidlicki, a spokesman for think-tank Open Europe, said: "Tusk has economically liberal and pro-free trade instincts, sensible views on energy policy, and will be sensitive to the concerns of the UK and other non-euro countries and it is significant that he committed himself to addressing UK concerns.
"However, he is likely to oppose fundamental changes to EU free movement - something David Cameron will be under huge pressure to tackle in his potential EU renegotiation. More immediately, Cameron will hope his early support for Tusk will help to land the UK a prime job in the new European Commission."
The pressure on Mr Cameron to deliver meaningful reform of the EU was underlined this week when one of his MPs dramatically defected to Ukip.
Douglas Carswell accused the Prime Minister of merely seeking cosmetic changes so he could campaign to stay in the union in the referendum - slated for 2017 if the Conservatives are in power.
The strong words from Mr Tusk - who described the prospect of a British exit as a "dark scenario" - suggests he could be an important ally.
However, Downing Street believe he is focused on curbing abuses of freedom of movement, such as inappropriate access to welfare, rather than fundamental changes to the principle.
According to another translation the president-elect - who was unanimously backed by EU leaders - also said: "Many of the suggestions put forward for EU reform are sensible, we can work together to eliminate any welfare abuse by EU migrants."