Asda fish counter sales have risen sharply following the launch of a new cook-in-the-bag product, which cost £1m to develop.

Customers are buying 30% more wet fish thanks to the oven-ready foil bags, which allow them to combine a portion of fish with a nob of flavoured butter. Shoppers pick the cut of fish themselves and can choose from a range of butter flavours, including unsalted and Thai-style.

The product was designed to appeal to customers who want the health benefits of fish but find preparation of the raw product unenjoyable, an Asda spokesman said. "Customer research shows many are reluctant to eat fish because of the aroma, texture and perceived hassle of cooking," he said. "Our foil bag allows customers to get their fish from store counter to oven, without once having to prepare or touch it."

Sales in the two weeks since the product's launch have also been helped by a TV campaign featuring footballer Ian Wright.