Upmarket UK retailer Waitrose achieved its biggest share of the country's grocery market in the last three months, according to the latest data from Kantar Worldpanel.

Waitrose accounted for 4.7% of UK grocery sales in the 12 weeks to 30 September, up from 4.4% in the corresponding period last year.

The retailer's sales increased 9.9%, outpacing a market up 3.9%. "It has also been a record-breaking period for Waitrose, posting its highest ever share of 4.7% and building on the strong performance we have seen over the past decade," Kantar Worldpanel director Ed Garner said.

The Kantar data showed Sainsbury's increased its sales by 5.6%, taking its market share from 16.2% a year ago to 16.5%.

"Among the big four supermarkets, the stand out performance is from Sainsbury's. Its sponsorship of the Paralympic games has clearly borne fruit, boosting its sales in this period. The retailer has also been bolstered by its Brand Match promotion and the ongoing relaunch of its own label range – both of which are proving popular with consumers," Garner said.

Asda's market share inched up from 17.4% to 17.5% although Tesco and Morrisons saw their share eroded.

Aldi sales rose 25.4%, while Iceland Foods' sales were up 9.2%. Garner added: "The growth of the discounters and Iceland continue to be a strong feature of the market. In particular, Aldi has been delivering growth in excess of 20% per year since mid-2011 – clear evidence that value for money allied to a growing emphasis on product quality is encouraging shoppers to increase their loyalty to the outlet."