Alan Barnden of the Environment Agency: "Flows have slowed to a trickle"

New figures show that after a long period of improvement, water quality in the drier parts of the country has started to decline.

The worst affected areas are the south of England and East Anglia and the agency, previously known as the National Rivers Authority, blames the increasing pollution on lower rainfall.

Oxygen levels in many rivers are worryingly low

It says this has caused river flows to reduce, which in turn slows the dilution of sewage and industrial waste.

Testing of rivers such as the Granta in Cambridgeshire found the amount of dissolved oxygen fell to worrying levels.

Oxygenation is vital for fish and other wildlife, such as frogs and toads, and there are fears global warming could make the problem worse in the future.

New homes in the countryside have increased pressure on rivers

Although levels of ammonia - left behind by sewage treatment - fell to a record low in East Anglia the overall quality of water was down because of the low flows.

Dr Mike Hulme, of the University of East Anglia, says the agency's report shows the cost of global warming and he says such problems will become increasingly common.

Dr Mike Hulme: "The environment is sensitive to climate change"

There are also fears the continuing pressure on housing in the south and east will force the government to allow more homes to be built in the countryside. Areas like Kent, Essex and Cambridgeshire are already suffering from high levels of water extraction which has reduced some rivers to a trickle.

There is better news for rivers in Wales and the north and west of England where rainfall levels have been consistent.