A DWINDLING band of creatures native to England's waterways could be under threat from the spread of their bigger American cousins, conservationists have warned.

Alarm bells have been ringing after three of the foreign crustaceans were spotted in the River Derwent, near Blaydon.

The arrival of the aggressive signal crayfish in rivers such as the Wansbeck and the Aln could spell disaster for the English white-clawed variety of the lobster-like animals.

They carry a fungal disease known as crayfish plague that has already driven out the native species in most parts of the south of England over the last decade with only small pockets remaining in areas such as Somerset.

The warning from the Environment Agency comes after organisations such as angling clubs and conservation groups have been told that special measures have been put in place to stop them spreading.

Conservation officer Anne Lewis said: ''At the moment, the signal crayfish seem to be present in the Derwent in low numbers but it is essential that we take every step to prevent their spread. The American species of crayfish is bigger, more aggressive and out-competes our native crayfish.''

She added: ''They have a weapon of mass destruction. The American one has a fungal disease which it carries the way in which a human has athlete's foot but it really isn't a problem. However, this disease is completely fatal to our native crayfish. It only takes one signal crayfish to wipe out a whole river.''

Anglers have been asked to follow simple disinfection procedures such as thoroughly drying angling equipment when moving between rivers to stop the disease spreading.

Officials also want to ensure that people do not introduce the signal crayfish to the Aln or the Wansbeck themselves.

In one incident in the south a group enjoying a barbecue by a river decided to release some into a river rather than eat them - with terrible consequences for the English species already living there. It is thought that those seen in the Derwent may have been introduced for breeding as the American species are often cultivated for the table.

The Environment Agency warned that it was illegal to keep or release into the wild any of the foreign species without a special licence from Defra.

The white-clawed crayfish have been in Britain since the last Ice Age but the new arrivals were only introduced in the early 1970s.

Their arrival has proved devastating to crayfish in the south with crayfish plague beginning to spread from the 1980s. Since then tens of thousands of the home species have been wiped out, or around half of the population