But firms win praise for high standards.

WATER treatment works in York were found to have small traces of a harmful bug in its supply by the Drinking Water Inspectorate (DWI), it was announced today.

Acomb Landing treatment works, run by Yorkshire Water, is one of eight sites treating river-derived water, which undergo continuous monitoring for cryptosporidium - a micro-organism which can sometimes cause diarrhoea and vomiting.

The tenth DWI report said they found minute traces of cryptosporidium in three samples taken in April and five during May.

Small traces of the bug were also found at a treatment site in Elvington.

But Claire Jackson, regional inspector for the DWI, reassured the public that the traces detected were well below the new treatment standard, with no reported illnesses in the local community.

She said: "This nasty little bug can come from cattle or humans. All treatment works holding river water without storage are considered at risk from it, so we have carried out continuous monitoring under new regulations.

"Acomb Landing has not exceeded our treatment standard despite traces being found. There is no public health risk associated with it. Our monitoring provides water companies forewarning and is a useful remedy. We are the first country in the world to bring this level of monitoring and regulation on cryptosporidium."

The new regulations create a new criminal offence with a maximum fine of up to £5,000 in magistrates' courts and an unlimited fine in crown courts, when treatment standards are breached.

Dr Anita Hatfield, of the North Yorkshire Health Authority, said the parasite was not a notifiable disease but they would certainly be told if there were outbreaks of illness associated with it.

She said: "There are so many different organisms which can give you diarrhoea it might often go undetected. The water companies have very strict guidelines and water from the tap is generally safer than bottled water."

A spokeswoman for Yorkshire Water said: "We are delighted that further improvements have been made to water quality in the area. As the results show, we have ensured that stringent safety levels have been maintained at all our treatment works."

The DWI report generally praised the water quality in Yorkshire and the Humber as the "best ever", with Yorkshire Water highlighted for achieving an improvement in overall quality.