The Environment Agency has called for urgent action after tests revealed that a third are in poor condition - reduced to a trickle or awash with pesticide residues.

Tests are underway on the Lambourn - which runs through Shaw and Speen - and the Pang, from Compton to Pangbourne. The tests are being run on 40 chalk rivers suffering from low flow, problems and to measure the impact of climate change.

But the Agency wants more help from West Berkshire Council and local MPs, as well as farmers and wildlife groups.

EA chairman Sir John Harman said: "Our chalk rivers are a valuable part of the English landscape, but they are currently under huge pressures. While much work is being undertaken to protect some chalk rivers, much more needs to be done.

"We need policy-makers, environmental regulators, businesses and local communities to take up the challenge of restoring this fragile ecosystem. Let us work together to ensure that generations to come are able to enjoy the unique heritage that chalk rivers represent."

Low flows are caused by abstraction to provide London with enough drinking water, but there is usually enough from the huge underground reservoirs beneath the Berkshire Downs.

However, as the Lambourn flows into the Kennet and Avon Canal, the water level can drop significantly from narrowboaters leaving canal locks open - a common sight in the summer months. The Environment Agency is expected to report back on boat traffic levels imminently.

The report also highlights an invasion of foreign plants and animals in our chalk rivers, pointing to Japanese knotweed, American mink and American crayfish for a corresponding crash in native species.