Four out of five private water supplies in the UK fail to meet EU drinking standards and put more than half a million people at risk of E.coli, a BBC investigation has found.

The study, by Radio 4's Farming Today programme, warns that the prevalence of the potentially fatal disease is rising, and it is entering the water course.

A leading microbiologist has described the situation as a "mess", and called for a tightening of the current regulations on water quality testing.

Dr Norman Simmons, a former member of a government food safety advisory panel, says more needs to be done to ensure that drinking water supplies are safe.

Water testing

In Britain there are at least 300,000 private water supplies.

The majority are in the countryside and they can range from a well supplying a single farm to large businesses. Some even supply hospitals.

They are defined as water that is not provided by a licensed water undertaker, in other words, a water company.

The extent of water testing appears to rely upon the number of people using it.

The regulations state that if a private supply provides water for one house then it may never be tested - and even if there is an adjacent campsite or caravan park it is only once a year.

If it supplies 500 people - and that could be a small village - then it need only be tested twice a year.