English salmon could be wiped out by a Norwegian parasite which has already devastated rivers in Scandanavia.
The tiny Gyrodactylus salaris is just half a mm long but it is lethal to Atlantic salmon.
It has been responsible for the total loss of wild salmon populations in some Norwegian rivers and the only way to kill it is by poisoning catchments.
The fisheries charity Salmon & Trout Conservation UK said they were concerned that the parasite could be brought into British rivers on the bottom of a canoe or fishing rod and is urging anglers and canoeists to keep their equipment clean.
“The best way of making sure that the UK and Ireland remains free and to protect our rivers from yet another devastating invader is to stop Gyrodactylus salaris from getting here in the first place.," said Paul Knight, CEO of Salmon & Trout Conservation UK.
"Live fish movements are already banned from infected countries, but anglers, canoeists, and those managing our rivers can play their part by observing the basic biosecurity rules of - Check, Clean and, Dry all equipment which might transport the parasite.
"The best contingency plan is to ensure that we never have to use one."
The charity said that Britain's rivers and chalkstreams were under increasing threat from non-native invasive species such as American Signal Crayfish, Himalayan Balsam and Demon shrimp.
They are currnetly carrying out detailed scientific monitoring of 22 rivers in England and Wales down to find out how far the invasive species have spread.
Mr Knight added: “ Crucially, we do not know how badly our rivers are being damaged by non-native species.
"Under the current system this could be a tragedy waiting to happen, which is why local anglers, catchment managers and conservationists are so vital in our battle to keep rivers clean and healthy.”