Parents should not allow under-fives to touch animals at petting farms, a microbiologist has said amid E.coli fears involving four sites.
Prof Hugh Pennington said they were the most likely people to touch animals but the hardest to get to wash their hands.
The Department of Health (DoH) said it was not changing its guidance - contact was okay if good hand hygiene followed.
Two petting farms in Surrey, one in Nottinghamshire and one in Devon have shut attractions owing to E.coli fears.
Prof Pennington said that parents should not "abandon the idea of visiting" petting farms, but they should "think very hard" about letting children under five touch the animals.
Hand-washing was "absolutely crucial" to protect visitors to petting farms, and under-fives "haven't learned how to do it yet".
It's so good for the children - one would hate to get to the stage where we had to stop them doing it
Miranda Stevenson British & Irish Association of Zoos & Aquariums
"These very young kids are the most likely part of the population to get complications if they get infected," he added.
But the DoH said its advice remained the same - children could pet animals but should practice good hand hygiene.
"Direct contact is not what presents the risk, it's what happens afterwards. If you use good hand hygiene and other measures, you hopefully shouldn't get it," a spokesperson said.
"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."
Current guidance was under review following the recent E.coli outbreaks, the spokesperson added.
And Miranda Stevenson, director of the British & Irish Association of Zoos & Aquariums, said there were strict licensing regulations for zoos and there were always either gels or washing facilities were animals could be handled.
"The message to parents is to make sure that children wash their hands or put gel on their hands, after they have handled animals," she said.
"It's so good for the children - one would hate to get to the stage where we had to stop them doing it."
Hundreds of members of the public have e-mailed the BBC News website with the majority expressing opposition to a ban on the petting of animals.
Forty-nine cases of E.coli have been linked to a Surrey farm
Prof Ron Cutler, an expert on infectious diseases at Queen Mary University of London, said more needed to be done to enable children to keep visiting petting farms and zoos.
Operators should think about giving visitors nail brushes to clean their hands after touching animals, he said.
Prof Pennington, who has led inquiries into E.coli outbreaks in central Scotland and south Wales, said: "Clearly I think the public expects that we have a really good look at the guidelines.
"And also to look at the way the guidelines are being implemented - it's all very well having guidelines if people are not following them."
World of Country Life, in Exmouth, Devon, voluntarily closed its petting areas and deer train ride following infections in three children who visited the farm last month - although the farm has not been confirmed as the source.
Four people became infected after visiting White Post Farm at Farnsfield, Nottinghamshire, though no direct link has been confirmed there.
Forty-nine cases of E.coli have been linked to Godstone Farm in Surrey, with its sister farm - Horton Park Children's Farm in Epsom - also closing because of hygiene concerns.
The Health Protection Agency says that, while infection risks are reduced by sites being well regulated and run, there would always be some E.coli risk from farm animals and their environment.
Whether to stay away from such attractions is "a decision for parents based on a balance of risks", its advice says.
Nine children are being treated in hospital with Godstone Farm-linked infections; one is due to be allowed home later on Saturday, with the others remaining in a stable condition, according to the HPA.
Lab tests have identified the most serious E.coli strain, 0157, which can cause serious kidney damage in a small proportion of cases.
There are about 20,000 cases of E.coli reported in England, Wales and Northern Ireland every year but last year just 950 cases in England and Wales were the 0157 strain.
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