"Shoppers are being reminded to thoroughly wash mixed salad leaves amid concern that this food could be the source of an E. coli outbreak that has killed two and infected more than 150 people in the UK.
Some people will have no symptoms, but others can develop serious complications and will need medical help.
Public Health England says the strain involved is likely to be imported, possibly from the Mediterranean area.
To date, it has been informed of 151 cases - 144 people in England, six in Wales and one in Scotland.
Of these, 62 needed hospital care and two patients died.
Most of the cases of the outbreak in England were clustered in the South West.
Dr Isobel Oliver from PHE, said: "All food sample results to date have been negative for E. coli O157 - but it's important to be aware that where food has been contaminated with E. coli O157, it is not always possible to identify the bacteria on food testing.
"As an additional precautionary measure, we have advised a small number of wholesalers to cease adding some imported rocket leaves to their mixed salad products pending further investigations."
People can help protect themselves from possible infection by washing their hands before eating and handling food and by thoroughly washing vegetables and salads that they are preparing to eat (unless they have been pre-prepared and are specifically labelled "ready to eat").
E. coli O157 is found in the gut and faeces of many animals, particularly cattle, and can contaminate food and water.
Outbreaks of O157 are rare compared with other food-borne diseases.
PHE was first alerted to the outbreak at the end of June.
A spokeswoman for Health Protection Scotland said: "We are supporting Public Health England and working with them on this issue.
"We would encourage people to follow the advice that has been issued.
Avoiding E. coli infection
Wash hands thoroughly after using the toilet, before and after handling food, and after handling animals
Remove any loose soil before storing vegetables and salads
Wash all vegetables and fruits that will be eaten raw
Store and prepare raw meat and unwashed vegetables away from ready-to-eat foods
Do not prepare raw vegetables with utensils that have also been used for raw meat
Cook all minced meat products, such as burgers and meatballs, thoroughly
People who have been ill should not prepare food for others for at least 48 hours after they have recovered