Health chiefs have issued an urgent hygiene warning after two people died and another 149 were struck down with food poisoning thought to be caused by salad leaves.
Cases of E. coli 0157 have been reported throughout England, Scotland and Wales, with south-west England worst affected.
Sixty-two people are known to have been treated in hospital since the outbreak began.
Experts from Public Health England (PHE) have been trying to identify the source of the infection noticed that a number of the victims had eaten salad leaves including rocket before becoming sick.
However, the cause is yet to be confirmed and officials are not ruling out other foods.
Testing of samples using state-of-the-art whole genome sequencing (WGS) technology suggests the bacteria concerned has probably come from overseas, with enquiries centring on the Mediterranean region.
The Food Standards Agency is assisting PHE with efforts to track down the source of the outbreak, but tests have so far proved negative.
Some retailers have been advised to stop using rocket from overseas in ready prepared mixed salads until further notice.
Experts are underlining the need to take sensible precautions to prevent food poisoning.
Symptoms of E. coli 0157 infection can include mild to severe diarrhoea and stomach pains. The illness can be spread by poor hand and toilet hygiene.
Director of PHE’s field epidemiology service Dr Isabel Oliver said officials are closely monitoring cases throughout the country and called for the public’s help to help control the outbreak.
“We continue to stress the importance of good hand and food hygiene practices at all times,” she said. “We urge people to remove any loose soil before storing vegetables and thoroughly wash all vegetables including salads that will be eaten raw unless they have been pre-prepared and are specifically labelled ‘ready to eat’.
“It’s also vital to wash hands thoroughly using soap and water after using the toilet, before and after handling food and after contact with any animals and pets, including farm animals. Small children should also be supervised when washing their hands.”