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A vicious breed of crayfish has invaded Hampstead Heath ponds and begun nipping swimmers.

The red swamp crayfish, known for its aggressive behaviour and long claws, is multiplying so fast that 5,000 of them are believed to be in the pools.

Bathers in the men’s pool, where many swim naked, have been advised to wear clothes.

Regular Peter McCafferty, 43, said: “I’ve been seeing more and more crayfish being pulled out and I’m very wary of them.

“I’ve had a couple of nips on my toes in the past few months. But some people have been nipped in altogether more sensitive places, which doesn’t bear thinking about.”