EU president Herman van Rompuy warned Thursday that Britain will struggle to win support for its plans to renegotiate its membership of the European Union, and said that trying to exit the bloc could prove costly.
In a speech in London, Van Rompuy urged Britain to stay inside the 27-member group, saying it had a role to play in reforming the union and would have a stronger voice in world affairs than if it went it alone.
But the European Council president said there was no appetite among EU leaders for any major treaty change, insisting the focus for the coming years would be on restoring stability after the crisis in the eurozone.
"Leaving is an act of free will and perfectly legitimate but it doesn't come for free," Van Rompuy said at an event organised by the Policy Network think-tank.
Prime Minister David Cameron has been accused of trying to blackmail other EU leaders with his plans to take back powers from Brussels and put the new terms of membership to a referendum by the end of 2017, with an exit one of the options.
"I cannot speak on behalf of the other presidents and prime ministers, but I presume they neither particularly like it nor particularly fear it," Van Rompuy said of the policy.
He said that "changing the EU treaties is not a priority", a point reiterated by Dutch Foreign Minister Frans Timmermans at the London event, who asked that if there was no new treaty, then what will the British hold a referendum on?
Van Rompuy warned it would not be easy for Britain to leave the EU.
"It's not a matter of just walking out. It would be legally and politically a most complicated and impractical affair. Just think of divorce after 40 years of marriage," he said.
Simply the threat of a "Brexit" was already hurting ties with other EU leaders, he said, noting: "How do you convince a room full of people when you keep your hand on the door handle?"
Van Rompuy argued that Britain's interests were best served by campaigning for reform within the EU, particularly in terms of competitiveness and tightening the single market.
"It is best if Britain and the rest of Europe channel these changes jointly," he said. "For a country like the UK to make its voice heard in the world, Europe does not work as a damper but as a megaphone."
Speaking at the same event, EU Economy Commissioner Olli Rehn also urged Britain to focus on reforming the bloc instead of pushing for a weaker union.
Rehn said it was "in everyone's interests for Britain to be an active player... no one has ever scored goals sitting on the bench".
"I believe it is firmly in Britain's interest to use its energy for reforming Europe rather than trying to undo our community which would leave us all weaker," he told the audience of European experts, academics and business people.
"In a nutshell, why not focus on reform, rather than repatriation?"
Cameron's announcement in January of the referendum, widely seen as an attempt to calm eurosceptic members of his Conservative party, sent shockwaves through the EU as it tries to return to stability after the three-year crisis in the eurozone.
While the prime minister has said he supports a renegotiated EU membership for Britain -- which does not use the euro currency-- recent polls show a majority of Britons would vote to leave the bloc.
Timmermans said he agreed with some of Cameron's concerns about Europe, including the lack of democratic accountability, and made a strong case for Britain to stay within the EU.
"I call upon the British government to join us with other countries to reform the union from within, to force the Brussels bubble to look at itself," he said.