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* No need for treaty changes, Van Rompuy says in London
* EU exit would cost Britain dear
* Says euro zone has "artillery", but could be aftershocks (Recasts, adds quotes and outside comment)
By Guy Faulconbridge and Andrew Osborn
LONDON, Feb 28 (Reuters) - One of Europe's most powerful officials cautioned Prime Minister David Cameron on Thursday that leaving the European Union could cost Britain dear and that the bloc's other leaders do not want to renegotiate Europe's founding treaties.
European Council President Herman Van Rompuy said Britain had a chance to play a leading role in building the European economy now the euro zone had the "artillery" of economic tools to get itself out of the worst crisis in its history.
But Van Rompuy laced his speech in London's financial district with a clear warning to Cameron: Europe will not countenance any attempt by Britain to win an a-la-carte membership, picking and choosing which of the European Union's rules it will follow and which to reject.
"Leaving the club altogether, as a few advocate, is legally possible," he said. "We have an 'exit clause'.
"But it's not a matter of just walking out. It would be legally and politically a most complicated and unpractical affair. Just think of a divorce after 40 years of marriage."
"Leaving is an act of free will and perfectly legitimate but it doesn't come for free," he said in a speech to bankers and politicians at the Guildhall, an 800-year-old institution that is a symbol of British merchant power in the City of London.
Cameron has promised to try to claw back powers from the EU and put any new settlement to voters in an in-out referendum by the end of 2017, heightening fears that Britain could leave the club it joined 40 years ago, in 1973.
"The wish to redefine your country's relationship with the Union has not gone unnoticed," said Van Rompuy, a former premier of EU founding member Belgium. "I cannot speak on behalf of the other presidents and prime ministers, but I presume they neither particularly like it, nor particularly fear it."
Cameron, a Conservative who says he wants Britain to stay inside the world's biggest economic bloc, warned in a speech on Jan. 23 that the European public was disillusioned with the EU and that Britain needed a new settlement.
British opponents of the European Union say it is a doomed project which has been imposed on European populations by an arrogant elite and that Britain should seek to go it alone.
Van Rompuy said Europe's leaders could not afford complacency and that bold reforms were needed to strengthen the euro zone, but that the integration ahead did not require further treaty changes.
"I see no impending need to open the EU treaties," he said, adding that Britain's ambivalent relations with Europe were undermining its negotiating position.
"How do you convince a room full of people when you keep your hand on the door handle?"
Van Rompuy's insistence on no treaty change undermined Cameron's strategy for getting Britain a new type of EU membership, said former EU Commissioner Peter Mandelson.
"The prime minister has made great play of radical treaty change - which he believes is necessary, which will provide the vehicle for the repatriation of powers that he wants. President Van Rompuy seemed to shoot that fox," Mandelson told Reuters.
"If there is not going to be such a new treaty, I don't know what the alternative vehicle will be for what the prime minister wants: you cannot have a unilateral negotiation."
Van Rompuy said a club producing one fifth of global gross domestic product gave its members a clout they would not have alone. But he dodged a direct question on whether a provisional EU deal to cap pay bonuses would damage the City of London, a major contributor to Britain's economy.
He said the 2008 global financial crisis had exposed the weakness of the European common currency project and kept open the chance of further "aftershocks".
"Talk of imminent break-up has vanished. It is finally sinking in that the euro is here to stay, and that this is due to deep political determination," he said. "Even if there may be turbulence ahead, we have the artillery we need." (Writing by Guy Faulconbridge; Editing by Hugh Lawson and Alastair Macdonald)