Leaky pipes may be causing upset stomachs because they allow contaminated water to get into the system, it is being claimed.
Sheffield University's Prof Joby Boxall issued the warning after studying the risks posed by leaky pipes.
His team recreated a mains water pipe system and then damaged part of it to see what happened to the water.
They found that when a pipe leaks, it not only spits out clean water, but also sucks in dirty water.
"Many of us will have had a 'dodgy tummy' in the past that we couldn't quite explain, often putting it down to something we'd eaten. It now seems possible that some of these illnesses could have been caused not by food, but by water," lead researcher Prof Boxall said.
Although his study did not look at the specific health effects of the finding, several previous studies on groundwater near pipes have found them to contain harmful pathogens and viruses.
Leaks are common within the UK network, and usually pose no problem because the high pressure of water pouring through the pipes means water just leaks outwards rather than being sucked inwards.
But problems occur when the water pressure suddenly drops, for example when it is needed to be diverted away for firefighting, or as a result of a valve or pump failure.
When that happens, if the pressure of the surrounding groundwater is higher than that in the pipes, or if there is negative pressure (pressure lower than the atmosphere), then external water will be sucked in.
Too many leaks
Water companies are now trying to manage this risk better, Prof Boxall said.
He said it was not economically feasible for the water companies to stop all leaks, so they were instead concentrating on preventing the pressure changes which could allow contaminants into the water system.
At the simplest level, this would be a matter of, for example, having firefighters turning water hydrant valves at a slower rate so that the pressure does not suddenly drop.
Several water companies have now built their own versions of Sheffield University's experimental facility in order to train their staff and sub-contractors, Prof Boxall said.
Sue Pennison, principal inspector of the UK's Drinking Water Inspectorate, said she welcomed the study, adding that the water industry is aware of the risks of incidents of water depressurisation and the need to minimise their frequency.