"Come out for the EU", British CEOs told by business lobby group

LONDON, May 20 (Reuters) - Britain's top bosses shoulddefend membership of the European Union by telling voters it isthe best guarantee of prosperity, the head of the country'slargest business lobby group will say on Wednesday.

Prime Minister David Cameron has pledged to renegotiateBritain's ties with Europe and then give voters an in-outreferendum on European Union membership by the end of 2017.

Mike Rake, chairman of BT and deputy chairman ofBarclays, will tell 1,000 chief executives andpoliticians to turn up the volume on the benefits of theEuropean Union so that Britain shuns isolation.

"Business must be crystal clear that membership is in ournational interest. The EU is key to our national prosperity,"Rake, president of the Confederation of British Industry (CBI),will say at its annual dinner in London.

"Business has increasingly spoken out on this crucial issueand the time has come to turn up the volume," he will say. "Thequestion is not whether the UK would survive outside the EU, butwhether it would thrive."

The threat of a British exit, or 'Brexit', has alreadyprompted some businesses to warn of the consequences for theworld's fifth-largest economy and London, the only financialcapital to rival New York.

Deutsche Bank said on Tuesday it was reviewingcutting its British presence in the event of a Brexit.

Rake will tell Britain's leaders to build EU alliances tomake the union more competitive but will caution that EU treatychange - a current demand from London - is not essential.

"We support the Prime Minister's drive for a morecompetitive EU," he will say, according to extracts released byhis office. "The EU can be more competitive without the need fortreaty change."

Ahead of last year's referendum of Scottish independence,some senior business leaders said that breaking up the UnitedKingdom would hurt Scotland.

Leaders of oil giants Shell and BP andfinancial services heavyweights Royal Bank of Scotland,Standard Life, and Barclays, all warned againstScotland leaving the UK.

Companies operating in the car industry have already comeout in favour of the EU, though there is no centrally organisedpro-European campaign yet.

The chairman of construction equipment company JCB said onMonday that Britain need not fear an exit from the EU. (Reporting by Sarah Young; Editing by Guy Faulconbridge)