By Ian Walker

LONDON--J Sainsbury PLC (SBRY.LN) was the clear leader over the Christmas period with a sixth consecutive monthly rise in market share, although Tesco PLC (TSCO.LN) managed to claw back a bit with a narrower fall of 0.1% compared with an average 0.4% drop over the year, suggesting that festive shoppers gave the retailer a welcome boost in the run-up to Christmas.

Outside the top four supermarkets, discount retailers Aldi and Lidl and up-market retailer Waitrose also increased market share in the 12 weeks, according to the latest data from Kantar, which monitors the grocery purchasing habits of 25,000 demographically representative U.K. households.

Sainsbury's market share rose to 17.1% in the 12 weeks ended Dec. 23, from 17.0%, and Waitrose's market share rose to 4.5% from 4.4%. Aldi and Lidl's joint market share rose to 6% from 5.2%. Separately, Aldi's share rose to 3.2% from 2.6% and Lidl's share rose to 2.8% from 2.6%.

Asda, a subsidiary of Wal-Mart Stores Inc. (WMT), saw its U.K. market share fall to 17.3%, from 17.5% in the same period a year earlier.

Tesco's market share slipped to 30.5% in the 12 weeks ended Dec. 23, from 30.6% in the 12 weeks ended Dec. 25, 2011, an improvement on its performance throughout 2012 when the average share drop was 0.4%. Morrison's share fell to 12.0% from 12.4%.

Edward Garner, director at Kantar Worldpanel, said the polarization of the market is highlighted by consumer spend levels which were widely anticipated to drop this year.

"While 47% of shoppers did reduce their spend in the lead-up to Christmas, 48% of shoppers increased their spend by 4.5% showing that 'two nations' continues to be a key feature of the grocery market", Mr. Garner said.

On Monday Wm Morrison reported a 2.5% decline in same-store sales excluding VAT and fuel in the six weeks ended Dec. 30, broadly in line with expectations, and avoided a profit warning by maintaining tight cost control.

Morrisons is the only major supermarket without an online grocery offer or an established convenience store network, the dearth of which is beginning to impact growth potential, while competition from discount stores like Aldi and Lidl are eating away at all the quoted retailers.

Sainsbury and Tesco are due to report Wednesday and Thursday, respectively, and are expected to have enjoyed positive sales growth.

The British Retail Consortium's monthly retail-sales monitor showed same-store sales--which exclude sales from shops that opened or closed during the preceding year--rose 0.3% on the year in December after rising 0.4% in November.

Total sales, which includes sales at stores that have opened in the past 12 months, rose 1.5% on the year in December, compared with an increase of 1.8% in November.

--Kathy Gordon and Ainsley Thomson contributed to this article.

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

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