By Ian Walker

LONDON--German retailers Aldi and Lidl continue to increase their market share in the U.K. with record sales in the 12 weeks ended April 26, taking both share and sales from the big four supermarkets, according to the results of a survey released Wednesday.

For the 12 weeks, Aldi's market share rose to 5.4%, from 4.7%, while Lidl's rose to 3.8%, from 3.5%. Sales in the period rose 15.1% and 10% respectively, according to Kantar, which monitors the household grocery purchasing habits of 25,000 demographically representative households in the U.K.

Despite this rise, the growth is slower than in recent months, suggesting that the discounter momentum is starting to slow a little, Fraser McKevitt, head of retail and consumer insight at Kantar Worldpanel said.

Outside the big four, the market share of up-market retailer Waitrose remained flat at 5.1% over the 12 weeks, while sales rose 1.5%.

The market share of number one retailer Tesco PLC (TSCO.LN) fell to 28.4% in the period, from 28.8% in the comparable period ended April 27, 2014, while Asda's share fell to 16.9%, from 17.3%.

J. Sainsbury PLC (SBRY.LN), which earlier reported its first fall in underlying profit in a decade, saw its market share fall to 16.5%, from 16.6%. Wm Morrison Supermarkets PLC (MRW.LN) market share fell to 10.9%, in the period from 11%.

All the top four retailers saw their sales slip in the period, with Asda showing the biggest fall of 2.2% and Sainsbury's falling the least at 0.2%. Tesco sales fell 1%, while sales at Morrisons were down 1.1%.

"Growth in the market has declined thanks to a record low for grocery price deflation... Lower costs are the result of both falling commodity prices and the ongoing supermarket price war, with all major retailers offering cheaper like-for-like goods," Mr. McKevitt said.

J Sainsbury group sales, excluding value added tax, edged down 0.7% in the 52 weeks ended March 14 to 23.78 billion pounds ($36.12 billion). Underlying pretax profit, which strips out one-time items and pension expenses, dropped 15% to GBP681 million but beat analysts' consensus estimate of GBP659 million, according to the company.

On April 22 Tesco reported the steepest ever full-year loss for a British retailer amid a raft of charges, capping the most tumultuous year in the grocer's 96-year history. For the year to Feb. 28, Tesco's pretax loss was GBP6.38 billion, compared with a profit of GBP2.26 billion a year earlier. The company logged charges of GBP7 billion, including GBP4.7 billion tied to the impairment of fixed assets.

Saabira Chaudhuri in London contributed to this article.

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