There are reports that a number of fish have already been killed and extraction points for household water have been closed as a precaution.

The full extent of the pollution will not be known until Monday morning and the Environment Agency is investigating.

Cefn Mawr firm Flexsys admitted diluted waste liquid from a storage tank had leaked from the site into the River Dee, via Trefnant Brook at 1420BST on Sunday.

An Environment Agency spokesman said the incident was "very serious" but early indications suggested a major disaster had been averted.

The River Dee is one of the most closely monitored waterways in Europe.

Environment Agency spokesman John Rowlands said: "It is obvious that a large amount of this effluent has reached the river but it appears it was diluted.

" The company said it had started its emergency plan and suspended manufacturing operations and directed "all available resources" to protect the river.

A company spokesman said: "This morning, we will work with officers from the Environment Agency to conduct a full investigation to establish both cause of the leak and to properly assess the environmental impact."

The River Dee is part of a network of lakes and rivers than supplies water to more than two million people in north Wales and along the border with England.

It is also an extremely important wildlife habitat for salmon and other species.

The Countryside Council for Wales has recently proposed declaring the whole length of the river - from Bala to the Dee Estuary - a site of special scientific interest.

The Dee Valley water company will close its abstraction point at Bangor-on-Dee on Monday to protect supplies.

The company says it will use clean water in storage and customers will not be affected.

Major leakage

Welsh Water, which has an abstraction station at Chester, says the pollution is currently "a long way away" and there is no need to shut down the pumps.

Environment Agency spokesman Martin Watkins says 225,000 litres of diluted industrial effluent are thought to have entered the river.

It is believed the pollution leak includes the compound phenol, which can give water an unpleasant taste.

Previous spills

In 1984, the River Dee became heavily contaminated with phenol - a number of people became ill and the water supply was contaminated for several weeks.

In recent years the river has suffered a number of pollution incidents, including one in July 2000 when 100,000 fish suffocated, but the source of that leak was never traced.

Last May, a pollution incident killed around 150,000 juvenile salmon at Maerdy, near Corwen.