By Ian Walker

LONDON--Discount retailer Aldi's market share in the U.K. has risen to a record 4.8%, nearly catching up with upmarket retailer Waitrose which stands at 4.9%, while former darling of the sector Tesco PLC (TSCO.LN) continues to fall.

As well as Tesco, the other casualty among the big four is Wm Morrisons Supermarkets PLC (MRW.LN), while J. Sainsbury PLC (SBRY.LN) and Asda shares remained static, according to Kantar Tuesday, which monitors the household grocery purchasing habits of 25,000 demographically representative households in the U.K.

"Waitrose has continued to resist pressure from the competition, testament to its policy of maximum differentiation, and has grown sales by 3.4%. This figure is well above the market average and thereby has lifted its market share," Edward Garner, director at Kantar Worldpanel said.

Both Aldi and Lidl, which maintained its record market share at 3.6%, have been boosted over recent years as shoppers look to get more for their money in a difficult economic environment. Combined, their market share has risen to 8.4%, compared with 6.8% for the 12 weeks ended July 21, 2013.

Tesco's market share has fallen to 28.9% in the 12 weeks ended July 20, from 30.3% for 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 groups to reduce prices in a fight to retain customers.

Morrison's share has fallen to 11%, from 11.5%, while Sainsbury's share was maintained at 16.6%. Asda, a subsidiary of Wal-Mart Stores Inc. (WMT), also maintained its market share at 17%.

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

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