Sainsbury's "Brand Match" programme tots up prices of comparable brands in shoppers' trollies. If shoppers could have paid less at Asda they receive a coupon for the difference.

A spokeswoman for Sainsbury's said the move reflected customer research which showed that "Asda is seen as the benchmark on price."

The move forms part of a prices strategy from Sainsbury's that has seen it lower the regular prices of products across its grocery business.

The spokeswoman said Sainsbury's had lowered base prices on about 1,200 lines over the last 18 months. She declined to say what the firm's total investment in prices had been over that period.

Sainsbury's, which is battling with Asda to be Britain's No. 2 grocer, saw a nine-year run of quarterly sales growth come to an end in the fourth quarter of its 2013-14 financial year and posted another quarterly decline in sales in the first quarter of its 2014-15 year.

Second quarter sales data will be published on Oct. 1, the first sales numbers to be presented by new Chief Executive Mike Coupe, who succeeded Justin King in July.

Industry data published on Tuesday showed Sainsbury's sales fell 1.8 percent in the 12 weeks to Sept. 14 in an overall grocery market growing at its slowest rate for more than 20 years.

On Monday Tesco was rocked by an accounting scandal, sending its shares sliding.

(Reporting by James Davey; Editing by Mark Potter)