(ShareCast News) - Sainsbury's has overtaken Asda to become the number two supermarket after Tesco, according to data released by market researcher Kantar Worldpanel.Despite a fall in sales of 0.3%, Sainsbury's had edged its market share of 16.5% ahead of Asda, which now stands at 16.4%.Kantar said Sainsbury's has returned to its position as the nation's second largest supermarket for the first time since January, boosted by non-food sales, its Sainsbury's Local outlets and faster market growth in the south of the country, where it operates a larger number of stores.Overall, sales across the 'big four' supermarkets were sluggish, up 0.8% in the 12 weeks ending 19 July from a year ago, with stronger growth being enjoyed by the smaller retailers.Aldi grew by 16.6% while Lidl saw growth of 11.3%, meaning both have moved to new market share highs of 5.6% and 4.0% respectively. Morrisons, meanwhile, was the best performer among the big four retailers, although sales fell back by 0.1%. Sales at Tesco fell by 0.6% and at Asda by 2.7%. At Iceland sales were up by 3.0%, coinciding with its recent 'Power of Frozen' advertising campaign.Clive Black, head of research at Shore Capital, said: "The Kantar update does not throw out many shocks or surprises to our minds. Accordingly, as we characterise above, we see a sector still struggling with the compression of top-line momentum by deflation and the challenge of industry price investment. Whilst so, expecting as we do further pricing & marketing activity when the folks return from their holidays, we also see a little more predictability and perhaps calm in the sector."