There are also fears children may mistake the plant for something which is harmless such as common hogweed.
Mike Duddy, of the Mersey basin Rivers Trust, claimed the weed was the most dangerous in the country.
He said: “It is now, without a shadow of a doubt, the most dangerous plant in Britain.
“Most people know about foxglove and deadly nightshade but not as many know about giant hogweed. Co-ordinated action is the only way to get rid of it. “In the 1990s there was very little of it. The proliferation has got worse every year.”
The Canal River Trust spends almost £100,000 trust to remove the plant.