Gatwick Airport has been fined £30,000 for polluting a river, killing more than 5,000 fish.

They died when detergent used to clear rubber off the runway was mistakenly diverted into the River Mole.

The alarm was raised after a dog walker discovered more than 50 dead fish just hours later.

Environment Agency officials immediately searched the river-banks to measure the extent of the problem.

They found dead and dying fish while parts of the river were frothing.

Workers also identified a strong citrus smell from the detergent around the watercourses and ponds surrounding the airport.

In an attempt to prevent the pollution spreading, a dam was constructed to pump contaminated water to a purpose-built drainage system. The following day the pollution had cleared after heavy rain.

Among the dead fish were 14 species including roach, perch, bream and a pike.

The airport spent more than £10,000 restocking the river.

Gatwick Airport Ltd, which was prosecuted for the blunder by the Environment Agency, admitted a charge of causing polluting matter to enter the river in September 2002.

Richard Banwell, prosecuting for the Environment Agency, told Lewes Crown Court it was difficult to estimate how many fish were killed but the figure was believed to be between 5,000 and 8,000.

The court was told the accident happened during routine cleaning of the main runway during the night.

The work needed to be carried out to remove grease and rubber from the runway to ensure the safe landing of aircraft. The work was usually done using high-powered water jets but the company had successfully carried out a trial with a new method using detergent, with the approval of the Environment Agency.

The detergent was sprayed on to the runway, left for 20 minutes and then agitated using machinery.

Low pressure water jets were used to hose the water away through the surface water drainage system.

However, that night a higher concentration of the chemical was used and soon a large amount of foam was covering the runway, posing a danger to landing aircraft.

Mr Banwell said: "Quite a huge problem developed.

"The foam was very slippery, too slippery to stand up on. If it had remained on the runway, aircraft would not have been able to land."

Extra equipment was quickly brought to the area to clear the runway before it opened for flights at 5am.

The decision was made to sluice the contaminated water into Monk's Yard, an area of the airport where there was a gully believed to connect to the airport drainage system.

But the water drained into Crawter's Brook and then into the River Mole.

Mr Banwell said the agency described the company as being environmentally careful but, he said, the pollution was caused by "inadvertent but very careless actions" in failing to check where the water would drain to.

Gerard Forlin, defending, said the company was "absolutely distraught" for causing the pollution.

Gatwick Airport Ltd, which has no previous convictions for environmental breaches, was ordered to pay £3,548 costs.