The Drinking Water Inspectorate says the standard of domestic supplies has improved because of a record number of prosecutions against water companies last year.

Its annual report showed water companies met a 99.78% compliance standard for drinking water quality tests.

For the first time the inspectorate published a league table showing which companies in England and Wales have the best quality drinking water.

Top marks for overall quality between 1996 and 1998 went to Essex & Suffolk, Severn Trent, Thames, Yorkshire and Anglian while Northumbria showed a worsening trend over the two-year period.

Rusty water

Michael Rouse, chief inspector at the Drinking Water Inspectorate said the results for 1998 confirmed the high quality in England and Wales.

But he admitted complaints and prosecutions for discoloured "rusty" supplies were on the up. This was due to renovation of water mains which led to more iron oxide in the water.

"Although high iron levels are not a health risk the water is unfit for human consumption and wholly unacceptable to consumers," said Mr Rouse.

"During the year, there have been a number of successful associated prosecutions and I will continue to take a tough line in response to these failures."

New rules on bug detection

Continuous sampling for cryptosporidium - a bug that causes diarrhoea - has also improved standards and water companies that do not treat their supplies properly are liable for criminal prosecution.

New rules come into force on Thursday requiring companies to carry out risk assessments to identify those sites where there is a significant risk of the bug.

Mr Rouse said the number of tests complying with the drinking water standards and which did not meet with the standards in 1998 was down to one eighth of the figure for 1992 showing the effectiveness of the enforcement system.

Environment Minister Michael Meacher welcomed the results.

"I am particularly pleased to see the result on compliance with the stringent pesticide standard in which 99.99% of the 651,683 tests met the standard, with only a few isolated samples not meeting the regulatory limit," he said.

"This is virtually 100% and the highest level of compliance of all the key parameters."