By Ian Walker

LONDON--Discount U.K. retailers Aldi and Lidl have maintained their record market shares as more shoppers flock to their stores, although both Asda and Waitrose have also seen their shares rise.

Combined, Aldi and Lidl's market share for the latest 12 weeks ended Aug. 17 rose to 8.4%, from 6.8% in the same period last year, mainly thanks to 53% of households shopping at either outlet over the past 12 weeks, Kantar--which monitors the household grocery purchasing habits of 25,000 demographically representative households in the U.K.--said Wednesday.

In addition, upmarket retailer Waitrose, and Asda, a subsidiary of Wal-Mart Stores Inc. (WMT), saw their market shares rise to 4.9% and 17.2% respectively, from 4.8% and 17.1%, Kantar said.

"Asda and Waitrose have achieved growth with differing strategies," Edward Garner, director at Kantar Worldpanel, said. "Asda has pushed its 'price lock' strategy to keep prices on everyday essential items low, while Waitrose is running competitive offers on home delivery alongside offers for myWaitrose card users allied to its overall quality and provenance positioning."

With the exception of Asda, the big four are feeling the squeeze as Tesco PLC (TSCO.LN) and Wm Morrisons Supermarkets PLC (MRW.LN) market shares remain under pressure, while J. Sainsbury PLC (SBRY.LN) has suffered a small drop in share, Kantar said.

Tesco's market share fell to 28.8% in the 12 weeks ended Aug. 17, from 30.2% in the same period last year. Tesco has been battling to boost flagging sales amid a changing U.K. retail environment, where the discount chains have been forcing the U.K.'s big supermarket chains to reduce prices in a bid to retain customers.

Morrison's share fell to 11% from 11.3%, while Sainsbury's fell to 16.4% from 16.5%.

Write to Ian Walker at ian.walker@wsj.com

Subscribe to WSJ: http://online.wsj.com?mod=djnwires