The white-clawed crayfish is the UK's only native freshwater species
Thousands of dead crayfish have been found in an Essex river sparking fears from environmentalists a plague could wipe out the native species.
The remains were found in the River Colne in Colchester and the Environment Agency is analysing samples.
The fungal disease Aphanomyces astaci, commonly known as crayfish plague, is thought to be the cause of the deaths.
A similar outbreak occurred on the nearby River Waveney in Suffolk in October last year.
The dead species is the Turkish crayfish, introduced to the UK by the restaurant trade.
But the Environment Agency is concerned the disease could spread to other river systems and harm the native white-clawed crayfish.
The native species is already under threat from the American signal crayfish, immune to the crayfish plague, which were introduced to the UK in the 1980s.
The Environment Agency urged river users to heed the potential presence of the deadly disease which is not only spread by affected crayfish but also equipment such as fishing nets and boats being transferred from infected waters.
Julia Stansfield, fisheries, recreation and biodiversity officer for the Environment Agency, said: "This second outbreak is really worrying for the future of our native crayfish.
"Without a change in people's behaviour, it seems only a matter of time before one or more populations of native crayfish is wiped out.
"We urge all river users to clean and dry or disinfect any equipment before taking it from one river or lake to another.
"A few careless acts could lead to the loss of this fascinating species from East Anglia."