Sainsbury has announced plans to wash and rinse all its bagged salads in natural spring water instead of chlorinated water.
The move follows press reports suggesting pre-prepared salads are low in nutrients and tainted with chemicals.
Sainsbury plans to wash half of its own label bagged salads, including its So Organic bagged salads, in spring water starting this month, and extend this across its entire own label bagged salads range in the next 12 to 18 months.
A spokeswoman said: "It keeps coming up at consumer groups that customers are worried about chlorinated water.
"There is a strong feeling that they would be happier if chlorine wasn't used in the washing solution, and that they want the product to be as natural as possible. Chlorinated water doesn't affect the taste of salads, and there are no health hazards associated with it, but this is a move towards more natural food preparation while simultaneously simplifying the cleaning process."
David Piccaver, chairman of the British Leafy Salads Association, said: "Washing salads in chlorinated water is a process that's been around for a very long time. There haven't been any health problems to my knowledge. But I would be very surprised if other retailers didn't go down the same route as Sainsbury."
A spokeswoman for Tesco said: "We are currently looking at a number of natural alternatives to chlorine.
"Using natural spring water is just one of the alternatives that we are considering."