UPDATE 1-Asda's sales slump as big UK supermarkets struggle

(Adds table, details) LONDON, May 4 (Reuters) - Sales at Wal-Mart's British supermarket chain Asda fell 5.1 percent in the 12 weeks to April 24, the worst performance in a period when all four of the country's big chains saw sales declines, industry data showed on Wednesday. Market researcher Kantar Worldpanel said sales at market leader Tesco fell 1.3 percent, ending four months of improvement, while Sainsbury's and Morrisons saw falls of 0.4 percent and 2.6 percent respectively.