Although they have been sighted in parkland during the current heatwave, one naturalist has predicted that wild terrapins could cause greater problems in the future.
Joe Pecorelli, of the London Aquarium, said: "If global warming continues, and temperatures rise in this country, then the terrapins will be able to breed more easily." The EU last month restricted the importation of terrapins after lobbying by environmental groups.
Concern first arose following the Teenage Mutant Ninja Turtles craze in the late Eighties when terrapin ownership increased dramatically with an estimated 8 million being imported into the between1985-1990. Although only the size of a 50p piece when young, terrapins grow rapidly, often to the size of a dinner plate.
Many terrapins, which attack fish, frogs and newts, were then dumped by their owners but are now thriving in ponds and lakes.
"Most people didn't realise just how big they can grow," said Sharon Harrison, of Petpals, in New Malden, Surrey. "This means they can be very expensive as bigger tanks have to be bought to accommodate them. This has led to people abandoning them in the wild."
There are estimated to be 800 terrapins scattered over more than 200 sites nationwide. The largest single community is in Roath Park Lake in Cardiff, which is thought to be home to at least 100. Gavin Jones, ranger at Roath Park, said: "They are a national problem. The winters haven't been killing them off... They can attack fish, frogs and newts and there is no natural predator for them as yet."