Quality checks on private water supplies used by businesses or serving more than 50 people found 11 per cent did not comply with expected standards.
A total of 3.6 per cent of the population get their water from a private source, not provided by Scottish Water, but many more people rely on them when visiting more remote and rural parts of Scotland.
A total of 46,470 tests on private water supplies were carried out in 2017.
But the proportion failing checks for E.coli - which can cause stomach pains, vomiting and nausea - has not changed in the last three years, leading to concerns that not enough is being done to make private drinking water supplies safe.
Drinking Water Quality Regulator Sue Petch said: “I am concerned with the poor quality of private water supplies, particularly the number that tested positive for E.coli and the health risks that its presence indicates.
“There is much that people responsible for a private water supply can do to protect water sources and ensure that there is an appropriate and robust treatment process in place together with a plan to ensure that this continues to be the case under all circumstances.”