Red-eared terrapins can live for up to 40 years

The creature, thought to be a red-eared terrapin, was seen in the river at Polson, near Launceston.

Two officers from the Environment Agency (EA) tried unsuccessfully to catch the 12in (30cm) animal, but it managed to evade capture.

Their popularity as pets grew with the 1990s Ninja Turtle television series, but many are now being abandoned.

It is illegal to abandon these creatures, but people get bored and they end up dumping them Doug Herdson, National Marine Aquarium

Paul Gainey, from the Environment Agency, said the officers had been able to establish it did not appear to be distressed.

He told BBC News: "The decision was later made not to try to catch it, given that it was in a rather wide and deep part of the river and it seemed to be absolutely fine. How it got there is the key question."

Red-eared terrapins are an alien species which can live up to 40 years.

Doug Herdson from the National Marine Aquarium in Plymouth said: "Unfortunately, terrapins can attack the fish population and they can also carry salmonella.

"It is illegal to abandon these creatures, but people get bored and they end up dumping them.

"If people don't want their terrapins they should contact the British Chelonia Group, but they shouldn't release them."

Last year the group, which looks after terrapins, turtles and tortoises released about 40 of the creatures in a specially created lake in Italy.