Airport bosses are facing a fine of more than £20,000 for polluting a river and killing more than 5,000 fish.

Gatwick Airport Ltd yesterday pleaded guilty to a charge of allowing polluting matter to enter the River Mole in September last year.

Janet Fedrick, prosecuting on behalf of the Environment Agency, said detergent which had been used to clear rubber off the runway flowed into the river via Crawters Brook, to the south of the airport.

She told Haywards Heath magistrates that fisheries officers thought the detergent had resulted in the death of about 5,200 fish and that 14 species had been affected.

"Going by the number of large fish which died, including pike, few fish would have survived," she said.

"It is quite likely that 75 per cent of the fish in the area were killed.

"We would class this as a category one type offence. It was not deliberate or intentional but the pollution still took place.

"Generally speaking, Gatwick Airport is a very environmentally friendly company but on this occasion things went wrong."

Gerry Forlin, defending, said things went wrong when airport staff tried to clear the runway of a rarely-used detergent which had unexpectedly foamed up and created a dangerous, slippery surface.

They used water hoses to clear the foam in preparation for landings but this caused effluent to flow into the brook.

However, he said it was no excuse for the airport's failure to deal with the run-off.

"This was an extremely shocking and demoralising event for this company. They really do take environmental issues extremely seriously. We also realise we fell well short of the mark on this occasion."

Magistrate Marti Vaughan referred the case to Lewes Crown Court for sentencing.

She said her powers, which meant she could only impose a fine of up to £20,000, might not be enough.

Legal guidelines in corporate responsibility cases suggest fines must be big enough to have an impact on the company.