Eight more cases of E.coli are being linked to a petting farm in Surrey.
The number of people affected by the O157 strain of the illness connected to Godstone Farm now stands at 57.
Ten children are in hospital, five of whom are being treated in London, although all are described as improving and none is seriously ill.
Godstone Farm closed last Saturday and its sister farm, Horton Park Children's Farm in Epsom, is also shut because of "unsatisfactory" hygiene arrangements.
Meanwhile, the owner of another farm with possible links to an E.coli incident has claimed the risk of infection has been "blown out of proportion".
The World of Country Life in Exmouth, Devon, closed its petting farm and deer ride on Friday on the advice of the Health Protection Agency (HPA).
Tests carried out
Three children who visited the farm while on holiday in Devon last month have the E.coli O157 infection.
Nigel Lee said tests would be carried out on his animals on Monday.
The farm has not been confirmed as the source of the infection, but Mr Lee said he was happy to close the two areas "as a precaution" while the HPA investigated.
An HPA spokesperson said the three children were believed to be "recovering well" from the E.coli infection.
The World of Country Life was the fourth farm to be closed or partially closed in recent days and it prompted an expert in microbiology to urge parents not to allow under-fives to touch animals at petting farms.
The Department of Health said a committee was looking into the possibility of changing future guidance on whether young children should have contact with animals at petting farms.
Currently it does not advise against contact but advocates thorough hand washing.
A spokesman said: "The risk of infection from E.coli 0157 through petting farm animals can be prevented by following everyday good hand hygiene measures.
"Ill health following a visit to an open farm is unusual even among children and these risks need to be balanced against the benefits for a child's education and development that arise from contact with animals.
"In the light of the current outbreaks of E.coli 0157, associated with visits to open farms, the advisory committee on dangerous pathogens is being asked to review the current guidance and will advise on the need for additional precautions."
Mr Lee said, as a father himself, he would always advise that children's hands were cleaned after touching animals.
"There has been a risk for some time and people have known that, but I just think it's being blown out of all proportion in the last few weeks, which is a shame."
Four people also became infected with E.coli after visiting White Post Farm at Farnsfield, near Mansfield, Nottinghamshire, though no direct link has been made.
Earlier in the week, the HPA apologised for delays in its handling of the outbreak at Godstone Farm and announced that an external investigation would be carried out.
Two-year-old twin boys, Todd and Aaron Furnell, were among those admitted to hospital.
The youngsters, from Kent, suffered acute kidney failure following the E.coli outbreak at Godstone Farm, which was closed last Saturday.
The HPA said the first case had come to light on 27 August but it later emerged that the agency had received a report of two cases in the previous week.
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