Health officials defended their decision tonight to allow a farm, which was involved in what was described as one of the largest outbreaks of E coli in the UK spread by animals, to stay open despite children falling sick.

Godstone farm in Surrey was only closed last Friday after several more cases of the illness were reported. It is thought the outbreak started as early as 8 August.

Managers at the farm, which lets children pet and feed animals, were told on 3 September to stop visitors having contact with high-risk animals by closing the barns and sandpits. But the farm was allowed to stay open. Tonight a Health Protection Agency (HPA) spokeswoman said: "This action [stopping visitor contact] has been taken in previous incidents and is both a proportionate and effective response for the scale of incident at that stage."

It was only when a fresh case emerged last Friday that the farm was asked to close the following day. The HPA spokeswoman said that it was uncertain how that person became infected, but action was needed "to protect the public from further exposure".

At least 36 people have become sick, with 12 children aged between 18 months and 10 years being treated after falling ill.

Four children remain seriously ill in hospital, of whom three are being treated in isolation at East Surrey hospital in Redhill. They all have complications arising from an infection that can lead to kidney failure, especially in the young.The HPA said that it could not rule out the possibility of further children becoming ill due to the bug's delayed incubation period.

Neil Wilson, whose nephew Tommy is seriously ill, said: "It's been a living hell. It seems quite surreal going in 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."

The HPA said tonight that one seriously ill child has improved.

Those infected have E coli O157, a pathogen which first appeared in Britain in the 1980s and to which children and older people are especially vulnerable. Like other strains, it can be transmitted through contact with animals.

Professor Hugh Pennington, an expert in bacteriology, said an outbreak of this size was unusual and food was a possible route of the infection. Pennington said it was unclear when the number of cases would peak. "The incubation period can be as long as 12 days. It depends on what caused the infection and whether the cases can be tracked back to one day, or if transmission was ongoing up until the day the farm was closed."

He said he would be surprised if the number of cases rose significantly. He added that there were about 1,000 cases of E coli infection in England every year.

Godstone – which keeps cows, pigs, sheep, llamas and rare poultry – attracts up to 2,000 visitors a day during school holidays. Health authorities are advising anyone who became ill since visiting the farm on 8 August to seek medical advice.

The farm's manager, Richard Oatway, said: "We have taken the decision to close the farm until the authorities have finished their investigations. These have been very thorough and are still ongoing. We will not reopen the farm until we are satisfied that we have got to the root of the problem and have put in place suitable control measures." In a statement read to BBC News he added: "All the staff at the farm are very upset about the E coli outbreak and hope all the children make a full and speedy recovery." He said the farm was told in August that there might have been an outbreak.

A Tandridge district council spokesman confirmed that the farm had been open last week. He said: "Measures to reduce the risk of the infection spreading were put in place by the farm last week."

• This article was amended on 14 September 2009. The original referred to the E coli strain as 0157. This has been corrected.