Global warming could be hitting water quality in the Thames Valley, a report by the Environment Agency suggests.

The agency's most recent figures for water quality in England and Wales in 1997 show a drop in water quality across the country.

The figures for the Thames region show that water quality was about 20 per cent lower last year than in 1996.

Pollution levels are measured against the figures for 1990, and while water quality across the country is still better than it was eight years ago, it has slipped back from the cleanliness standards reached in 1996 and 1995.

Dr Jan Pentreath, the agency's chief scientist, said the latest findings showed a clear weather-related effect, with the cleanest rivers and canals to be found in the coolest parts of the country and the dirtiest in the warmer south-east.

The implication was that if global temperatures continued to rise water quality could continue to drop.

"Monitoring is beginning to pick up a north-west/south-east divide for trends in water quality," she said.

"This could turn out to be a long-term problem and indicative of the sorts of effects climate change could have."

The Environment Agency report said that the problem had disguised continued improvements created by better sewage treatment by water companies and industries.

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