An outbreak of an "unusual" type of salmonella which has led to nine people being admitted to hospital may be linked to cooked ham from independent butchers, health experts have said.
The Food Standards Agency (FSA) said the outbreak of Salmonella typhimurium resulted in 21 confirmed cases in Wales and a further 30 in England.
The victims ranged from a seven-month-old baby to an 87-year-old.
No contaminated ham has been found but one supplier has voluntarily withdrawn all ham products as a precaution.
The outbreak was first detected in Wales in August and centred mainly in northern areas such as Conwy and Gwynedd. Additional clusters are being investigated in England in the Thames Valley, Leeds, Cumbria and Stafford areas.
Officials said it was the biggest salmonella outbreak since 2006 when Dairy Milk recalled millions of chocolate bars following fears that they were contaminated.
The FSA said there was a potential common link between one supplier of cooked ham to independent butchers.
A spokesperson said: "Testing has been carried out on ham supplied to a number of butchers identified in the investigation.
"No trace of salmonella has been found but other hygiene issues were identified that led to one supplier voluntarily withdrawing certain batches of ham."
Salmonella poisoning is usually caused by eating contaminated raw or undercooked food such as meat, eggs, and dairy products. Symptoms can include diarrhoea, stomach cramps, vomiting and fever.