A spawning ground for trout in West Sussex has been devastated by a chemical spill which could threaten numbers of the fish for years to come.

Fungicides and herbicides poured into the Bignor stream, at Bignor, after a road accident yesterday afternoon, killing fish and spreading into the River Arun.

The cause of the spillage has yet to be established but it is thought a car hit a chemical container which had fallen off a mobile spraying unit on a road near the stream.

Firefighters from Petworth, Arundel and Chichester wore breathing masks and protective clothing and poured 1,000 litres of water into the stream to dilute the Cycocel and Starain XL chemicals, which are deadly to fish and can cause eye irritation in humans.

The Environment Agency launched an investigation into the incident and warned people to take care where the Bignor stream enters the Arun.

Jo Sangster, of the Environment Agency, said: "This is a big chemical spill. We are treating it very seriously."

She said the scale of the spillage was not yet known but there were "some dead fish in the Bignor stream along with some live ones."

She said pollutants which had reached the tidal River Arun should be diluted by today but said: "There is a great deal of aquatic life in the area."

Trout come to spawn in the tributaries of the Arun, including the Bignor stream, at this time every year.

The young fish swim out to sea and return to the Arun the following year. Any interruption in the cycle could badly affect numbers returning to the river, which also contains roach, dace, chub, perch, and carp.

Fred Puttick, chairman of Pulborough Angling Society, which controls three miles of the River Arun, described the spillage as worrying, but added: "It depends on the volume that went in, the volume of water and how it dilutes.

"Any pollution is serious in our eyes. Conservation is a thing we look after and respect.