Seven people have been infected with a strain of E coli after visiting an area of parkland, public health officials have confirmed.
The Health Protection Agency (HPA) said it was working with Birmingham City Council to investigate the outbreak at Sutton Park in Sutton Coldfield, West Midlands.
Of the seven patients identified as being infected with E. coli O157 in recent weeks, five were children under eight and at least one is known to have been hospitalised.
Advising parents that E. coli O157 can cause serious illness in young children, the HPA said the strain of the bacteria was often associated with contact with rural environments that may have been contaminated by animal faeces.
Dr Roger Gajraj, a consultant with the HPA's West Midlands East Health Protection Unit, said: "E. coli O157 can be very serious in young children and can in some instances cause kidney failure.
"The council is increasing hand-washing facilities at the park and the city council and HPA are issuing leaflets and displaying posters to warn visitors of the risks and advise on preventative measures.
"However to minimise the risk of infection parents may want to consider taking very young children to other parks in the local area."
Dr Gajraj added: "The best protection against E. coli is to always wash your hands, especially after contact with animals, after going to the toilet and immediately before eating.
"I would also advise cyclists and walkers to wash their tyres, footwear and their hands after visiting Sutton Park as an extra precaution."
Sutton Park, which covers around 2,400 acres and offers bridle-paths and cycling routes, is thought to attract around two million visitors annually.
Designated a National Nature Reserve, the former royal deer park is included in English Heritage's list of historic parks and gardens, and includes sites of archaeological interest.
Birmingham City Council's head of parks, Darren Share, said: "We are working closely with the HPA to control this outbreak and we have already taken a number of measures.
"But as the infection can be particularly serious for young children we feel it is appropriate at this stage to warn parents and families of the potential risks."