By Ian Walker

LONDON--J. Sainsbury PLC (SBRY.LN) has leap-frogged rival Asda to become the second-largest U.K. supermarket by market share for the first time since January, despite both share and sales falling, according to survey data released Tuesday.

For the 12 weeks ended July 19, Sainsbury's market share fell to 16.5%, compared with 16.6% in the 12 weeks ended July 20, 2014. Sales fell 0.3% to 4.16 billion pounds ($6.45 billion), according to Kantar, which monitors the household grocery purchasing habits of 25,000 demographically representative households in the U.K.

Asda, a subsidiary of U.S.-based Wal-Mart Stores Inc. (WMT), saw its market share fall to 16.4%, from 17% a year earlier with sales 2.7% lower at GBP4.15 billion.

"Sainsbury's has returned to its position as the nation's second-largest supermarket for the first time since January, boosted by non-food sales, its Sainsbury's Local outlets and faster market growth in the south of the country, where it operates a larger number of stores," Fraser McKevitt head of retail and consumer insight at Kantar Worldpanel said.

"Growth has accelerated at Waitrose, where sales have risen by 3.0%. Customers have taken advantage of the recently introduced 'Pick Your Own Offers' initiative to push market share up to 5.0%, an increase of 0.1 percentage points compared with last year," he added.

For the 12 weeks ended July 19, Waitrose sales rose to GBP1.27 billion and its market share rose to 5%, from 4.9% in the comparable 12 weeks last year.

Number one retailer Tesco PLC (TSCO.LN)'s market share fell to 28.5% in the 12 weeks, compared with 28.9% in the comparable period ended July 20, 2014. Tesco sales are down 0.6% over the period to GBP7.2 billion.

Wm Morrison Supermarkets PLC (MRW.LN)'s market share slipped to 10.9%, from 11%, while sales are 0.1% lower at GBP1.59 billion.

Outside the top four, German discounters Aldi and Lidl continue to see their market shares rise. The combined market share of Aldi and Lidl rose to 9.6% in the 12 weeks ended July 19, compared with 8.4% in the year earlier period. Aldi sales have risen 17% to GBP1.41 billion, while Lidl's sales are up 4% to GBP1 billion.

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

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