The latest annual report from the Drinking Water Quality Regulator (DWQR) shows that water from public supplies in Scotland is cleaner than it has ever been, but the quality from private sources is causing concern. 99.83% of 155,302 tests from taps in homes met regulatory standards in 2010.

This is the best result since Scottish Water was set up. In 2003, the year after the publicly-owned company was formed, 99.14% of samples passed, and in 2009, 99.78% achieved the standard.

However, 18.35% of samples taken from private water supplies in 2010 contained E coli, which can cause significant health problems in humans.

About 150,000 people in Scotland (3% of the population) receive water from a private water supply.

A DWQR spokesman said: "Households can be confident that the drinking water from their tap has been tested thousands of times each year, and that it meets some of the tightest quality standards in the world."

However, the spokesman said that people who use private supplies were risking their health. Many of these supplies have limited or no treatment. The main risk comes from micro-organisms, including E coli.

"Local authorities across Scotland are working hard to communicate to owners and users the health risk these private supplies pose, and offer support to make improvements," he said.

"Where improvements are being made it is vital that the newly installed treatment is properly maintained for benefits to be seen."