Thousands of children are at risk from E. coli because the farm pinpointed as the source of an outbreak was allowed to remain open for two weeks after it first fell under suspicion.

Godstone Farm, in Surrey, where children can pet and feed the animals, was finally closed at the weekend. Environmental health officials first visited it on August 28 and then again on September 3.

Twelve children under the age of 10 remained in hospital last night; three were said to be very seriously ill. A total of 36 cases have been confirmed by the Health Protection Agency (HPA) in what it said was Britain’s biggest E. coli outbreak spread by farm animals.

Up to 2,000 people, half of whom were children, visited the farm each day during the school holidays and similar numbers have visited over recent weekends.

The HPA, which said that the first known confirmed case dated from August 8, warned parents, nurseries and schools to be extra vigilant.

Professor Hugh Pennington, one of Britain’s leading microbiologists, said: “The consequences of this bug can be catastrophic in young children and it can be lethal.” It was unclear when the number of cases would peak, he said, adding that there was a 12-day incubation period for this O157 strain.

The professor said that the cause of the outbreak remained a puzzle. The bacteria were carried in animal faeces and could be picked up by hands, clothing and shoes. Most E. coli cases in Britain are caused by drinking contaminated water or eating infected meat.

Richard Oatway, the farm manager, defended the response yesterday. “We were told in August [that there might have been an outbreak] and we have taken the decision to close the farm [on Saturday] until the authorities have finished their investigations,” he said.

New information released by the HPA last night revealed that one person became infected with the bug after contact between visitors and the farm’s animals had been banned.

Godstone Farm’s sister premises in Epsom, Surrey, remain open. A spokesman for Horton Park Children’s Farm said that the only link between the farms was “purely financial” and there was no contact between animals.

Graham Bickler, the regional director of the HPA, said: “It is very likely that the source was animals at the farm. We know this organism is in their faeces but we need to find out how it got into the kids.” The agency defended its decision not to ask for the farm to be closed immediately, saying the scale of the problem was not immediately apparent.

Neil Wilson, an uncle of one of the sick children, said last night: “It’s been a living hell. It seems quite surreal going in and each day seeing him getting worse and worse.”

One mother who visited Godstone Farm two weeks ago said yesterday she was concerned that her 23-month-old daughter may have contracted the bug. Evelina Niedzwiedzka, 28, from Croydon, South London, said: “If the farm did know about it two weeks ago, I’m very surprised there were no warnings, especially because children are so vulnerable.”

Neil Wilson, an uncle of one of the sick children, told Sky News: “It’s been a living hell, it seems quite surreal going in and seeing him and each day seeing him getting worse and worse, feeding tubes and blood going in. It’s just awful, it’s been an absolute nightmare.”

A spokeswoman for the HPA said last night: “We have had no new cases reported today. Twelve children remain in hospital. Of those, three remain seriously ill. Six remain stable, and three children who were being looked after in paediatric units in London have recovered sufficiently to be moved to hospitals nearer their homes.”

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