All the cases reported so far developed symptoms before 4 June and the number of cases being reported is now falling, offering hope the outbreak may have ended.
Several food manufacturers removed some of their products from shelves as a precaution.
The Food Standards Agency said lettuce is the likely source, based on tests.
Darren Whitby, head of incidents at the FSA, said: "Earlier this month, we confirmed that several sandwich manufacturers had taken precautionary action to withdraw and recall various sandwiches, wraps, subs and rolls after food chain and epidemiological links enabled us to narrow down a wide range of foods to a type of lettuce used in sandwich products as the likely cause of the outbreak."
He said work continues to confirm this so that actions can be taken to prevent a re-occurrence.
"This remains a complex investigation and we continue to work with the relevant es and the local authorities to ensure necessary steps are being taken to protect consumers," he said.
As of 25 June, there have been 275 recorded cases of E. coli (STEC) O145 across the UK.
So far, 182 cases have been recorded in England, 58 in Scotland, 31 in Wales and four in Northern Ireland. The latter cases are thought to have acquired their infection in England.
At least 122 have been admitted to hospital for treatment since the start of the outbreak.
The overall number of affected could still rise because some samples from patients have yet to be tested, experts say, although rates are slowing.
E. coli are a diverse group of bacteria that normally live in human and animal intestines.
Some types are harmless but others can make seriously ill.
The variety in this outbreak, Shiga-toxin-producing E. coli (STEC) O145, can attack the gut's lining.
Symptoms usually take a few days to develop and can include:
Symptoms can last up to two weeks in uncomplicated cases. Most recover well but some - such as young children or with underlying health conditions - can become very unwell.
There is no specific treatment for E. coli infections.
should seek medical help if worried.
Copyright 2024 BBC. . Theis
