(ShareCast News) - Growth in the British grocery markets continued to be sluggish, with all the "big four" chains losing market share, according to the latest figures published by Kantar Worldpanel for the 12 weeks ending 16 August.Sainsbury's was the only one of the "big four" to post any growth, with sales there up 0.1% and marking the first increase since March, although the retailer lost its number two spot to Asda. Sainsbury's market share slipped 0.1 percentage point to 16.3%.Tesco, which is still in the top spot, was the second-best performer in terms of sales, with a 0.9% drop. Its market share fell to 28.3% from 28.8%.Meanwhile, Kantar said like-for-like sales at Wm Morrison fell 1.1%, while sales at Walmart-owned Asda dropped 2.5%. Morrison's market share declined to 10.8% from 11%, while Asda's fell to 16.6% from 17.2%.Waitrose, meanwhile, saw its sales push up 3.7% from last year on the back of its 'Pick Your Own Offers' promotion, while the Co-operative saw sales at its tills up 1.1%.The discount supermarkets fared the best.Fraser McKevitt, head of retail and consumer insight at Kantar said: "It's been another successful period for the discounters, with growth at Aldi accelerating to 18.0%. Lidl's sales have also risen, up 12.8%, taking its market share to a new high of 4.1%."As anticipated, Asda has retaken its position as Britain's second largest supermarket, despite a fall in sales of 2.5% and a 0.6 percentage point fall in market share. The retailer's greater focus on non-food items means its market share is traditionally higher in the summer, and it's expected that Sainsbury's will again become the number two retailer towards Christmas."