J Sainsbury's share of the UK supermarket sector remained unchanged for the first time in six months as it was hit by competition, according to researcher Kantar Worldpanel.Market share of the third-largest food retailer held steady at 16.5% in the 12 weeks ended July 7th. It was the first time it stopped growing since January. Kantar said Sainsbury, Tesco, Asda and Morrison have been under pressure as a result of long-term market polarisation.Aldi, Lidl and Waitrose now have a combined 11.5% share of the market, 3.2 percentage points more than four years ago."This trend has cut deeply into the available market share for the bigger retailers who are now having to compete for a contracting middle ground," said Edward Garner, Director at Kantar.However, Sainsbury is the only one among the 'big four' supermarkets to not lose share in the past year with year-on-year growth of 3.8%. While Morrisons is continuing to lose market share, the retailer's sales growth has shown progressive improvement this year, rising from the low of -1.7% in January to the current figure of 1.8%.The figures also demonstrate that price matching campaigns at the larger retailers have shed light on private label quality. "The latest price matching promotions from retailers have reduced the amount consumers are shopping around, with many people feeling that they can get the same prices at different retailers," Garner added."As a result we are seeing an increased focus on quality. Both Tesco Finest and Sainsbury's 'Taste the Difference' ranges are now growing strongly and Aldi's 'Like Brands only Cheaper' campaign and Lidl's 'Taste Test' are positioning their private products as direct competitors to major brands."Grocery inflation remained at 3.9% for the 12 week period ending, slightly higher than the market growth of 3.7%.RD