The number of cases is 161 (England 154, six in Wales and one in Scotland).
More than 60 people have received hospital care and two with E. coli O157 infection have died.
The outbreak is characterised by multiple small clusters linked to catering and residential care premises.
Association with mixed salad and rocket leaves
Investigations suggest there is a strong association with eating mixed salad including rocket leaves from catering establishments such as cafes and restaurants.
In response, a number of wholesalers stopped adding some imported leaves to mixed salad products, pending further investigations.
Samples have been confirmed as Shiga toxin-producing E. coli (STEC) serogroup O157 phage type 34, positive for the eae (intimin) and verocytotoxin 2 genes but negative for the verocytotoxin 1 gene.
The European Centre for Disease Prevention and Control (ECDC), the European Food Safety Authority (EFSA) and the European Commission are also involved in the investigation.
All results from sampling and microbiological examination of salad products have been negative for the strain of E. coli causing the outbreak.
WGS data analysis
Analysis of whole genome sequencing (WGS) data indicated isolates fell within a 5-SNP (single nucleotide polymorphism) cluster, a strain not related to those circulating in the UK bovine reservoir.
The outbreak strain was most closely related (>70 SNPs) to sequences identified in people reporting recent travel to the Mediterranean region suggesting the strain was likely to have been imported.
However, other European countries have not reported similar increases, and the strain has not been reported outside the UK.
The UK Food Standards Agency’s (FSA’s) investigations are focusing on distribution of mixed salad leaves to wholesale and not supermarkets.
As of 14 July, onset dates for primary cases ranged from 31 May to 5 July.
Cases were predominately female (119/158) and 15 were under 18 years old with ages ranging from one to 98 years.
Features of haemolytic uraemic syndrome (HUS) had been reported in seven cases.
Dr Isabel Oliver, director of PHE’s field epidemiology service, said: "As previously, we strongly advise maintaining good hand and food hygiene practices at all times, particularly for anyone affected in order to stop the infection passing to others.
“It is important to remove any loose soil before storing vegetables and to thoroughly wash all vegetables (including salads) that will be eaten raw unless they have been pre-prepared and are specifically labelled ‘ready to eat’.
“These measures may reduce the risk of infection from any E. coli contaminated vegetables, fruit and salad but will not eliminate risk of infection completely."