The Environment Agency says it has found widespread evidence that native plants are disappearing from the country's waterways and action needs to be taken to protect them.

As the plants go, so do insects and animals

Giant hogweed, Himalayan balsam and Japanese knotweed have been blamed for much of the damage as they take a stranglehold on banks.

Victorians introduced the three species as ornamental plants into English gardens but they have since escaped into the wild and proved ferocious in taking over wetlands.

The roots of a Japanese knotweed can tunnel up to seven metres into the earth.

Just a tiny amount of matter left behind after it is dug out can sprout a new plant.

Many chemicals used to kill these plants destroy other wildlife.

Concerted effort needed

Agricultural change is also to blame

The Environmental Agency's river habitat survey found that the Mersey in the North West has the largest number of all three weeds.

The agency called on local councils across the UK to work with it to stop the advance of the plants.

The report also said that increased building in rural areas and changes in agriculture meant that rivers are slowly losing other animals and plants which used to live on the banks.

Alistair Driver, one of the report's authors, said urgent action is needed to protect the character of the nation's water courses.

"Knotweed and the balsam are quite wide spread throughout the country," said Mr Driver.

"As a result, we lose the diversity that we used to have in the river banks.

"Things do change naturally but this is through man's influence. That is not always for the best.

"We are seeing the effects of man's activities in urban areas changing the character of river banks."

Survival of the fittest

Himalayan balsam is the tallest annual plant found in Britain, growing up to two to three metres in height.

It can produce up to 800 seeds which can remain viable for up to two years.

Giant hogweed, a native plant of the Caucasus Mountains between Russia and Turkey, has a stem and leaves covered in hairs coated with poisonous sap. Its seeds can remain active for up to 15 years.