Public Health England (PHE) said some of those affected had been treated in hospital after becoming unwell and that the outbreak appeared to be linked to the consumption of watercress. Thirteen of those affected are from England, with four in Wales and one in Scotland.
The Food Standards Agency (FSA) put out the Sainsbury's recall notice, which also affects the supermarket's salads that contain watercress, as a "precautionary measure" after the outbreak of E.coli O157.
Those who have bought the watercress since August 1 have been advised not to eat it.
A statement on the FSA website said no other Sainsbury's products are thought to be affected, adding: "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."
"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concern and so, although no traces of contamination has been found in our products, we have as a precautionary measure withdrawn six lines of pre-packed salad containing watercress from the supplier concerned.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."
PHE said the outbreak was mainly affecting people over 50 and that it was being investigated by itself and the FSA "as a matter of urgency".
Interviews with those affected "revealed a strong link to the consumption of watercress", with most saying they had bought it at Sainsbury's.
The particular strain of E.coli, called VTEC O157 Phage type 2 VT2, 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.
"The Food Standards Agency immediately contacted the retailer with the details of the outbreak and they promptly took action to withdraw the product."
In England, nine of the 13 cases are in people over the age of 50. Ten women and three men have been affected.
All of the five in Scotland and Wales are over 55. Two are women and three are men.
Confirmation of test results are being awaited for a number of other people, PHE added.
The products recalled by Sainsbury's are:
:: by Sainsbury's Watercress Leaves, 75g
:: by Sainsbury's Watercress, Spinach and Rocket salad, 100g
:: by Sainsbury's Watercress, Spinach and Rocket, 170g
:: by Sainsbury's Watercress twin pack, 2 x 35g
:: So Organic Watercress, 75g
:: So Organic Watercress, Spinach and Rocket, 100g