THOUSANDS of fish have been killed in a major pollution incident in the New Forest.

Now swimmers are being told to keep out of the Lymington River at Brockenhurst after raw sewage escaped from a Southern Water treatment works.

Effluent poured down a storm overflow and entered the river after a valve at the plant in Balmer Lawn Road developed a fault.

The sewage contaminated a one-mile section of the river. It is expected to have a disastrous impact on the picturesque waterway, which was recently stocked with trout.

The alarm was raised after dead fish were seen floating on the surface of the river near the B3055 Beaulieu to Brockenhurst road.

Environment Agency staff were sent to the scene and attempted to improve conditions by increasing the amount of dissolved oxygen in the water. Southern Water used tankers to remove incoming sewage while repairs were carried out to the valve.

Environment Agency team leader Dawn Theaker said: "The sewage leak has had a disastrous effect on the river.

"Low flows in the Lymington River have meant the impact of the sewage has been much worse. If there had been more water in the river it would have helped dilute the problem."

The incident, started on Sunday, was first thought to have killed about 500 fish but thousands are now known to have died.

An Environment Agency spokesman said: "We have taken samples and readings and are warning swimmers to stay out of the river.

"We never advise people to swim in rivers because of the potential hazards caused by strong currents and objects on the bottom. In this case there's the added danger of contaminated water."

Len Shearing, chairman of Wellworthy Angling Club, said the pollution was likely to have a "colossal" effect on the river. A Southern Water spokesman said: "We attended the scene as soon as we became aware of the problem and immediately launched a clean-up operation.

"The company has started an investigation to determine the cause."