A predatory Asian crab that makes its home in freshwater and is blamed for bank erosion has been found in a river linked to the inland waterway system.
Asian Mitten Crabs are blamed for river bank erosion
The Chinese Mitten Crab has been found for the first time in Norfolk.
Ecologists are now waiting to discover the scale of infestation in the River Great Ouse, near Downham Market.
The giant crab, named because of its large claws, has been eroding banks and killing native species in the Thames, Humber and Medway estuaries.
The discovery in the Denver Sluice raises fears that the crabs are on the move and could infiltrate the River Cam in Cambridge or even Milton Keynes in Buckinghamshire.
Crabs burrow into banks to make a safe haven for themselves from the flowing water. This riddles the bank with holes until it eventually collapses.
Environment Agency head ecologist Terry Clough said: "One was seen scuttling out of a net and we believe it is the first sighting of one there.
"The worry is that if there is a significant increase in numbers. In the Thames for some reason the population took off in the 90s and the effect has been quite serious damage and erosion of the banks.
"If the population in the Great Ouse increases to any great extent we would be properly concerned that there might be some damage to the river banks."
Mr Clough said there had been a report by a fisherman of seeing a Chinese Mitten crab near Earith in Cambridge where the waterway system could give it access to the River Cam, increasing its spread around the country.