Health watchdogs have warned people to thoroughly wash mixed salad leaves after the food item appeared to be the source of an outbreak of E coli food poisoning that has so far infected 151 people in Britain, leaving two of them dead.

Public Health England (PHE) said several of those affected were known to have eaten mixed salad, including rocket, before becoming unwell.

Symptoms can include bloody diarrhoea, stomach cramps and occasionally fever. People usually notice signs of the illness three to four days after they have been infected, but symptoms can start any time between one and 14 days after ingesting the bacteria and can last up to two weeks.

Testing of samples from those who had fallen ill also indicated the strain of E coli 0157 was likely to have been imported, possibly from the Mediterranean.

Facebook Twitter Pinterest A colorised scanning electron micrograph image showing a strain of E coli. Photograph: Janice Carr/AP

PHE officials said they had not ruled out other potential food sources for the outbreak, which has so far involved 144 cases in England, particularly the south-west, with six in Wales and one in Scotland. Two people have died and 62 people have needed hospital care.

Isabel Oliver of PHE said the body was working with the Food Standards Agency to trace, sample and test salad products grown in the UK and other parts of Europe. “All food sample results to date have been negative for E coli 0157, but it’s important to be aware that where food has been contaminated with E coli 0157, it is not always possible to identify the bacteria on food testing,” she said.

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A small number of wholesalers had also been told to stop adding some imported rocket leaves to their mixed salad products pending further investigations, Oliver said.

Outbreaks of E coli 0157 poisoning are relatively rare compared with other food poisoning diseases. The bacterium is found in the gut of many animals, particularly cattle, and can contaminate food and water.

PHE said people could help protect themselves from infection by washing their hands before handling food and eating, and by thoroughly washing vegetables and salads they were preparing to eat, unless these had been pre-prepared and were specifically labelled “ready to eat”.