London (AFP) - The Bank of England revealed it is researching the economic risks of Britain leaving the European Union, just as Prime Minister David Cameron began negotiations with EU leaders ahead of a referendum on membership.The central bank study, which could have a major impact on the debate over Britain's future within the bloc, was due to remain confidential but details were accidentally emailed to an editor at The Guardian newspaper.Cameron used an EU summit in Latvia to kick-start negotiations on changing Britain's terms of membership.