A highly contagious crayfish plague which is lethal to Britain's only native species has broken out.
The white claw is the UK's only native species of freshwater crayfish
The Environment Agency has confirmed aphanomyces astaci in the River Waveney that forms the Suffolk/Norfolk border.
This fungal disease is lethal to native white-clawed crayfish already under pressure from immune American signal crayfish introduced in the 1980s.
Native crayfish have been wiped out in many rivers by the much larger signals with whom they compete for food.
Over recent weeks hundreds of Turkish crayfish, another introduced species, have been found dead on the river near Beccles.
Fungal infection has wiped out large numbers of native and non-immune species since it was brought to the UK in the 1980s by the American signal crayfish.
Amanda Elliott, spokeswoman for the Environment Agency, said tests had confirmed an outbreak of aphanomyces astaci, the fungal disease commonly known as crayfish plague.
Measures are now being put in place to try to prevent any spread, particularly to rivers populated by the native British white-clawed crayfish.
Ms Elliott said: "What we are most concerned about is preventing the disease spreading to another river system. Luckily, there are no British white-clawed crayfish recorded in the River Waveney."
River users are being urged to thoroughly clean equipment before using it in other waterways, she added.
Signals are immune to the infection, which they carry on their shells, but white-clawed crayfish cannot fight the fungus which spreads through their body and eats them alive.
Remote bodies of water known as 'Ark' sites have been sought out to maintain the native species across the UK, such as the River Glaven in North Norfolk.
Julia Stansfield of the Environment Agency said: "We're really worried about the disease getting into other rivers and especially the Wensum where there's a good strong population of native crayfish still present.
"If it starts getting into other rivers as well it's possible that we could see no more native crayfish in East Anglia."