The damage may have been caused by as little as two spoonfuls of the organophosphate nerve agent which is used on lawns and golf courses and to protect crops and fruit from insects.

The spill has led to a “total wipe out “ of aquatic invertebrates on 10 miles of the river between Marlborough in Wiltshire and Hungerford in Berkshire.

It is thought to have passed through Thames Water’s Marlborough sewage works after entering further upstream.

Thames Water is working with the EA to find the source of the pollution which has struck the river at the height of the fly-fishing season.

The same pesticide was linked to a similar incident in the River Wey in 2003 and led to a significant fish kill on the Sussex Ouse in 2001.