Asda's problems continue to mount as the latest market data suggests the Wal-Mart owned supermarket chain is losing ground to all three of its main rivals.Asda's market share has fallen to 16.7%, its lowest level for 18 months, according to the latest figures from Kantar Worldpanel, the consumer research group.The supermarket chain's sales grew by 2.7% in the 12 weeks to 13 June, well adrift of Tesco, which grew by 3.7%, Sainsbury's, which notched up 4.4% growth, and Morrisons,which surged 6.1%.Asda's second place spot behind Tesco is now being threatened by Sainsbury's, which has increased its market share to 16.2% and cut the gap between the grocers to just 0.5%, the smallest gap between the two since December 2007.Tesco continues to dominate with a market share of 30.8%, while Morrisons is now up to 11.9%.Waitrose is the fastest-growing supermarket with sales up by 11.2% over the period lifting its share to 4.2%."It's interesting to see shoppers continuing to trade up, as demonstrated by Sainsbury's and Waitrose as well as Tesco Finest. As we emerge from the grip of recession, quality is back on the agenda. This has come to Asda's attention, with their latest drive to promote their prize-winning products," Edward Garner, Kantar Worldpanel's communications director, said.