The rare strain of E. coli O157 was identified by PHE's whole genome sequencing technology, and resulted in 161 cases of food poisoning, of which two led to deaths.
"As the latest date of onset in a primary case is 5 July, this outbreak has now been declared over," PHE said.
Investigations by PHE's epidemiological and microbiological staff working with the Food Standards Agency (FSA) and local authorities identified mixed salad leaves as the likely cause of the outbreak.
PHE incident director Professor Jeremy Hawker said: " We continue to work closely with the FSA to confirm the source of this outbreak and to take all the appropriate measures to protect the public."
He added: "We urge people to remove any loose soil before storing vegetables and thoroughly wash all vegetables and salads that will be eaten raw unless they have been pre-prepared and are labelled 'ready to eat'. These measures may reduce the risk of infection from any E. coli contaminated vegetables and salad but will not eliminate any risk of infection completely."