It may be a discomforting thought, but earlier this week a seagull flew over London carrying one of the razor-toothed fish in its beak, which it then proceeded to drop onto the deck of a boat on the Thames.

This, at least, was the theory put forward yesterday by the Environment Agency as it revealed Tuesday's discovery of a 10cm piranha, more than 5,000 miles away from its tropical home in the Amazon in south America.

The piranha, the world's most ferocious freshwater fish, was dropped onto the deck of the Thames Bubbler at Halfway Reach in Dagenham, east London.

It is thought a seagull scooped the fish out of the Thames, where it may have been dumped by someone who owned it as a pet. The cold water would probably have killed it before the intervention of the seagull.

The fish, one of a species which has been known to attack humans, was fresh and had only just died. Marks from a seagull's beak were still on its back.

Crew on the boat, owned by Thames Water and used to pump oxygen into the water to keep fish alive, suspected it was a piranha - but were rather confused as to why they had encountered it.

It was taken to London Aquarium to confirm its species. Paul Hale, curator of the aquarium said: "It is definitely a Red Bellied Piranha, but it would not survive in the low temperatures of the Thames, and we imagine it was probably released and then floated to the surface where it was picked up by one of the hungry seagulls and deposited onto the boat."

Red Bellied Piranhas (Pygocentrus nattereri) have short, powerful jaws lined with razor-sharp teeth capable of devouring 16cm of flesh with each bite. They hunt in shoals capable of stripping and eating even large prey in a matter of seconds.

Experts say their reputation for attacking humans is exaggerated, but it is thought a shoal of the fish devoured up to 300 people when their boat capsized and sank near Obidos in Brazil in September 1981. And piranha attacks on bathers have also been increasing in Brazil due to the damming of certain rivers.

But the fish cannot survive in temperatures below 15C for more than a few days - and the temperature of the Thames is currently 10C. So Mr Hale said anyone on the Thames worried that other piranha might be out there would be safe.

He said: "Piranhas are generally nervous and not the ferocious killers people think they are. They prey on weak and injured animals, including fish, birds and mammals, as well as carrion."

The dead piranha is being kept in deep freeze by the Environment Agency, which warned it was an offence to release any non-native species into the wild.

Piranhas have taste buds which cover their bodies so that they can deduce whether any passing fish is worth eating. It doesn't bear thinking about what such sensitivity must have made of the polluted Thames.