Dishwashing tablets are threatening the home of British fishing, the rivers Test and Itchen in Hampshire.
The chalk streams which feed into the two rivers, famed for fly fishing, are showing dangerous levels of phosphorus.
Phosphorus speeds up the growth of weeds as well as turning the crystal clear waters dark and murky. The rivers are believed to be the finest chalk streams in the world and are classified as Sites of Special Scientific Interest.
Mammals, birds, fish, including trout, chubb, tench, and bream, all enjoy the rivers’ waters.
The Environment Agency said the Itchen failed a recent chemical quality test, and officials expect it to fail again in 2015. This places the river in the "at risk" category.
Dr Steve Rothwell, from the Vitacress Conservation Trust, said: "The UK is unusual in Europe because many EU countries long ago banned detergent products with phosphorus and the UK never has.
"In Hampshire phosphorus is a big problem. You get too much algae growing in the rivers as a result and it starts to out-compete the other plants you want.
"We know septic tanks are a problem. The chalk streams are so clear because they lack phosphorus, but if you add any it's like adding fertiliser and you get all this growth."
Fertilisers running off the land into the waters also bring up phosphate levels.
Britain has 95% of all the world’s chalk streams, predominately located in the southeast of England.
The water quality of the Test is so superb that from the upper reaches it is used for washing and processing the paper which is used to produce British banknotes.
