The new research from ACNielsen, which is based on total till spending over the last 12 weeks, shows Sainsbury's market share has risen to 15.3%, marginally beating Asda's 15.2%. The company says a sustained strong market performance has driven the result. Tesco is still the market leader.
The improvement for Sainsbury's comes after it faced a run of brand-damaging problems last year as some of its stores in London ran out of fresh food. The problem was blamed on stock forecasting software following a £3bn investment in the supermarket's supply chain.
Sainsbury's has benefited by the introduction of new healthier product lines. It last month signed Jamie Oliver to front its advertising for another year.
Eleni Nicholas, group managing director for ACNielsen UK & Ireland, said: "Sainsbury's has been working hard to improve performance, with lower prices and improved availability, as recognised by its customers.
"In addition, we are seeing a trend towards healthy eating, and traditional Sainsbury's shoppers have always recognised the importance of a balanced diet."
The ACNielsen data is based on all purchases, barcoded and non-barcoded, brought back into the home by an in-home scanning panel of more than 10,000 households. The results include all items stocked at Sainsbury's and Asda outlets including grocery, durables and clothes.
The findings come as Asda plans an overhaul of its advertising, in which England and Manchester United footballer Wayne Rooney and his girlfriend Coleen McLoughlin will become the new faces of the company in a deal worth £3.5m.
Asda also dropped its long-running "Always low prices" slogan and Asda Price jingle at the beginning of March and plans to increase adspend to £45m. The move is designed to match the marketing spend of rival Tesco.
If you have an opinion on this or any other issue raised on Brand Republic, join the debate in the .