The Chartered Institute of Environmental Health (CIEH) took water samples from decorative water features across South Wales.
Of the 52 samples taken, 25 per cent contained E. coli levels exceeding recommended standards. None of the garden centres was able to produce cleaning schedules for the water features.
When asked, none of the garden centres' personnel was aware of the potential risk presented by the contaminated water.
"Decorative water features in garden centres or any other outlet are usually at a low level and readily accessible," said a CIEH representative. "Children are potentially at risk."
Most strains of E. coli are harmless and live in people's and animals' intestines, but some strains produce a powerful toxin and can cause severe illness.