The retailer's sales fell 2.9% in the 19 weeks to January 3 and by 0.3% over the six-week Christmas period.
Earlier it announced that it was launching a fightback by cutting prices on hundreds of its products.
Shoppers will see lower prices on household favourites from brands such as Hovis, Marmite and Coca-Cola as the company tries to win back market share from discount rivals such as Aldi and Lidl.