Marks & Spencer's Wisebuys range now accounts for 20% of food sales, 18 months on from the value lines' launch.

Executive chairman Sir Stuart Rose said M&S was developing additional ranges including 80 new confectionery products, as the food team battles to rejuvenate the business.

"The rate of change of innovation will increase," Sir Stuart said, speaking as M&S published a first-quarter trading update that showed like-for-like food sales down 0.5%. "This is about us reacting to the difficulties of last year, when we were sliding down a ski slope and giving customers damn good value, damn good innovation and damn good pricing."

He claimed M&S had seen 1% deflation in its food prices, suggesting volume sales were actually in growth. Total sales for the food business were up 2% by value, with group sales up 2.9% overall.