Sainsbury was the only one of Britain's top four supermarkets that achieved market share growth in the 12 weeks to August 18th, according to researcher Kantar Worldpanel.Total sales at Sainsbury grew 4.9% compared to a year ago, bringing the grocer's share of the £31.7bn market to 16.5%."Sainsbury's has continued to grow ahead of the market over the past 12 weeks, said John Coll, Director at Kantar Worldpanel. "It benefited from its support of the Paralympics last year and its growth has continued since then."Tesco and Asda, on the other hand, saw their market share fall to 30.2% and 17.1%, respectively from 30.9% and 17.5% last year.Morrison's market share shrank to 11.3% from 11.5%.Kantar said the big four have been hit by rising competition from discounters such as Lidl and Aldi as consumers cut down on costs amid a tough economic climate.Aldi's market share jumped to 3.7% and Lidl's gained a market share of 3.1%.RD