Tesco, Britain's biggest retailer, said on Tuesday that its chief executive Terry Leahy is to stand down in March 2011 after 14 years in charge and will be replaced by board member Philip Clarke. 'Sir Terry Leahy has decided, after an outstanding career at Tesco, to retire in March 2011 at the age of 55 after 14 years as CEO,' a company statement said. 'The Tesco Board has moved to implement its long-term succession plan, with Philip Clarke succeeding Sir Terry.' 