Britain's plans to renegotiate its relationship with the European Union lack support from other leaders, while an exit from the EU could prove costly, EU president Herman van Rompuy warned Thursday.
Speaking at a forum in London, Van Rompuy said that Britain would have a stronger voice in world affairs if it stays inside the 27-member group and compared a so-called "Brexit" to a divorce.
"Leaving is an act of free will and perfectly legitimate but it doesn't come for free," Van Rompuy said.
He also said that Prime Minister David Cameron's plans to shift powers back from Brussels and then put Britain's EU membership to a referendum before the end of 2017 would not influence other EU leaders.
"I presume they neither particularly like it nor particularly fear it," he said of Cameron's policy. "How do you convince a room full of people when you keep your hand on the door handle?"
He added: "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."
But Van Rompuy said 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."