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LONDON, Jan 9 (Reuters) - UK business leaders, includingVirgin Group founder Richard Branson and WPP chiefexecutive Martin Sorrell, warned Prime Minister David Cameron ina letter to the Financial Times on Wednesday that he riskshurting Britain's businesses if he proposes to exit the EuropeanUnion. In the letter, 10 signatories said more EU reform wasurgently needed, but Britain had to be careful not to call for awholesale renegotiation of its membership. "To call for such a move in these circumstances would be toput our membership of the EU at risk and create damaginguncertainty for British business, which are the last things theprime minister would want to do," the letter said. Other executives that signed the letter included Jan duPlessis, chairman of miner Rio Tinto , BritishTelecoms's chairman Michael Rake and Chris Gibson-Smith,chairman of the London Stock Exchange. Cameron, heeding growing Euroscepticism among Britishvoters, has said that he wants to revamp Britain's relationshipwith Brussels and is expected to set out his intentions in aspeech later this month. His increasingly vocal insistence on renegotiating termswith the EU has raised concerns that Britain may be heading foran exit, although Cameron has said he thinks Britain shouldremain part of the EU. On Tuesday, Ireland, which holds the presidency of the EUfor the next six months, urged Britain not to be too adversarialin pushing for change in its relationship with the EU.