keep toddlers from animals' call

Professor 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.

Closed voluntarily

That age group was "for all sorts of reasons ... the people most at risk", he said.

World of Country Life, in Exmouth, 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.

Professor Pennington 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."

Professor Pennington has led inquiries into E.coli outbreaks in central Scotland and south Wales.

Nine children are among those 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 Health Protection Agency (HPA).

Earlier this week, the HPA apologised for delays in its handling of the Godstone Farm outbreak and announced that an external investigation would be carried out.

The HPA had said the first case came to light on 27 August but it later emerged that the agency received a report of two cases a week before.