While two people in England have died within 28 days of being infected with the current strain, information from clinicians suggests “one of these deaths is likely linked to their STEC infection”, the UKHSA said.
Both individuals had underlying medical conditions and they both died in May.
At least 122 people have been admitted to hospital for treatment since the outbreak began.
Current case totals are as follows: 182 in England; 58 in Scotland; 31 in Wales; and four in Northern Ireland, although “evidence suggests” those four “acquired their infection in England”, the health body added.
As of 25 June, a further 19 cases had been reported in the latest seven-day period.
It brings the total number of confirmed infections across the UK to 275.
All currently registered cases showed symptoms before 4 June.
Those that are unwell should not prepare food for others and avoid visiting people in hospitals or care homes.
They are also advised not to return to work, school or nursery until 48 hours after their symptoms have stopped.
Darren Whitby, head of incidents at the Food Standards Agency, said the investigation “remains complex”.
He added: “We continue to work with the relevant businesses and the local authorities to ensure necessary steps are being taken to protect consumers.
“Although we are confident in the likely source of the outbreak being linked to lettuce, work continues to confirm this and identify the root cause of the outbreak with the growers, suppliers and manufacturers so that actions can be taken to prevent a re-occurrence.”