Sainsbury's has stressed that no traces of E.coli have been found in watercress and that ongoing tests are taking place on a precautionary basis.
The supermarket chain earlier announced that it was recalling all of its bagged watercress and salads containing watercress due to a potential association with an outbreak of E.coli 0157 which has made 15 people ill.
A Sainsbury's spokesperson said:
The Food Standards Agency has made us aware that a small number of people have fallen ill with a bacterial infection, and that one of their lines of investigation is watercress bought at Sainsbury's since 1st August.
Customer safety and the quality of our food are our overriding concern, and so although no traces of contamination has been found in our products we have as a precautionary measure withdrawn six lines of pre-packed salad containing watercress from the supplier concerned.
We are urgently testing all similar products and have to date found no indication of contamination. We will of course keep customers fully updated.
Customers who have bought any of these products from Sainsbury's since 1st August should not consume them. They can be returned to store for a full refund. We apologise for the inconvenience this may cause.”