Thirteen of those affected are from England, with four in Wales and one in Scotland. Some of the victims are being treated in hospital.

The Food Standards Agency (FSA) said the supermarket giant was also recalling salads containing watercress, adding that the move was a precautionary measure due to a possible association with an outbreak of E.coli VTEC O157. No other Sainsbury's supermarket products are known to be affected.

The FSA said: "(Sainsbury's) is advising people not to consume any of these products and to return them to the store they were purchased from for a full refund. Investigations by the Food Standards Agency, Public Health England and local authorities are continuing, and further information will be provided once it becomes available. Sainsbury's has informed the FSA that it is carrying out testing on all of its affected lines, but that no trace of E.coli O157 has been detected to date."

Public Health England said the outbreak was mainly affecting people over the age of 50 and interviews with those affected "revealed a strong link to the consumption of watercress," with most saying they had bought it at Sainsbury's.

The VTEC O157 strain of E.coli causes symptoms ranging from mild gastroenteritis to severe bloody diarrhoea, with blood poisoning and kidney failure in the most serious cases.

Dr. Dilys Morgan, head of the gastrointestinal, emerging and zoonotic diseases department at PHE, said: "VTEC infections can be very serious. Although all ages are affected, this outbreak is mainly affecting older people. Since children and the elderly are particularly vulnerable for the complications of VTEC infections, it is important that we took action as soon as possible.

"We have a robust surveillance system in place for VTEC and this helped us identify that watercress was the most probable food item which was causing the illness. We then followed this up with detailed food questionnaires and this confirmed that most cases had eaten it."

A Sainsbury's spokesperson said: "Although no contamination has been found in our products, we have as a precautionary measure withdrawn six lines containing watercress. We are testing all similar products and have 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. We apologise for the inconvenience this may cause."

The six products withdrawn from Sainsbury's shelves are:

• Sainsbury’s Watercress Leaves, 75g
• Sainsbury’s Watercress, Spinach and Rocket salad, 100g
• Sainsbury’s Watercress, Spinach and Rocket, 170g
• Sainsbury’s Watercress twin pack, 2 x 35g
• So Organic Watercress, 75g
• So Organic Watercress, Spinach and Rocket, 100g