The source of a major pollution incident on the River Kennet is still unknown.

Action for the River Kennet (ARK) volunteers first saw signs of the chemical spill on July 1 and it has devastated the insect and fresh water shrimp population between Marlborough and Hungerford. The Environment Agency believes the chemical is chlorpyrifos, a common agricultural pesticide, but investigators have not been able to identify where it entered the water.

ARK director Charlotte Hitchmough said: “We were hoping that Thames Water might have been able to pinpoint the source of the pollution but they haven’t been able to so far.

“They have been carrying out a lot of tests and continue to do so but so far nothing.

“A team from University College London is going to be looking at the long-term effects.”