THAMES, Britain's largest water company, has the worst record of failures in meeting tap-water quality standards, according to a government inspector's report published yesterday.

Just over one in every 20 Thames tap-water samples breached at least one of more than 30 limits set for different contaminants by the Government and the European Union, the Drinking Water Inspectorate's annual report discloses. Excess levels of weedkillers were to blame for most failures with one type, atrazine, above the limit in 38 per cent of samples where it was looked for.

The much smaller Essex Water company had the next worst record in 1993, with 3 per cent of total samples failing. Most of the drinking water suppliers managed under 1 per cent, with Chester Waterworks heading the league - less than 1 in every 1,000 samples failed.

Alan Nield, drinking water quality manager for Thames, said the company and its three million customers were disadvantaged by taking the bulk of its supplies from the rivers Thames and Lee, which were contaminated by pesticide run-off from fields and urban areas.

He said an expensive investment programme in advanced carbon filtration equipment was now beginning to pay off with a strong fall in pesticide limit breaches this year.

Both Mr Nield and Michael Rouse, the chief drinking water inspector, said there was no danger to public health because the limits were so cautious. The pesticide standard equates to only one part of pesticide per ten billion parts of water.

The inspectorate monitors the English and Welsh water companies' own sampling for bacteria and a wide range of other contaminants. Its report shows that in nearly one in 100 tap-water samples in Bournemouth Water Plc's area, faecal coliform, the bacteria associated with sewage, were found - the highest level for any company. The inspectorate said it was considering enforcement action to prevent a repetition.

Across England and Wales, lead levels above the 50 microgrammes per litre limit were found in 3 per cent of samples.

Lead is known to be highly effective at damaging nerves and the European Commission is considering lowering the limit value following a new assessment of lead's toxicity by the World Health Organisation.

But Mr Rouse, the chief inspector and a former senior officer with the former water authorities' research centre, said he was confident there was no lead danger in tap-water or any other health threat.

Overall, 98.9 per cent of the 3.5 million samples carried out by the water companies had met the standards. 'Tap- water is good and getting better, he said. 'But that's not the view of the public, who have been fed with ill-founded scare stories.'

Mr Rouse said he was worried to see 'vulnerable, impressionable' people who could ill afford bottled water scared into buying it. Tapwater was much more closely and carefully monitored than bottled water.

The Government had no plans to prosecute any water companies for supplying water unfit to drink, nor has it ever done so in its four-year history.