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A 'dolphin' has been captured swimming in the River Thames - leaving Londoners amazed.

British Olympic swimmer Andy Jameson, who won bronze at the 1988 Seoul Games, spotted the marine mammal emerging from the water in Richmond, south-west London.

Mr Jameson said he felt "mad" as he revealed his sighting of a dolphin in the Thames on Twitter .

But social media users were quick to point out the creature was more likely to be a porpoise, which have smaller noses and bodies and are more common in the capital.

(Image: Andyjamesonswim/Twitter)

Mr Jameson tweeted: "Ok .... you may all think I'm mad, but I just saw a Dolphin in the river Thames at Richmond."

Amazed Londoners suggested the animal was more likely to be a porpoise but added the swimmer was fortunate to see it.

(Image: Andyjamesonswim/Twitter)

Shaun Johnson wrote: "Wow very strange".

And Ben Underwood added: "Porpoise are smaller with a stubby nose. Very lucky to see it".

While another commented: "Great sighting. You've got the proof. A dolphin & a seal have been spotted in Teddington Lock before too."

(Image: Andyjamesonswim/Twitter)

In August 2015, a survey by the Zoological Society of London found marine life was thriving in the Thames with regular sightings of whales, dolphins, porpoises and seals among animals inhabiting the river.

Other strange sightings have also been reported.

Last April, footage appeared to show a mysterious giant creature lurking in the Thames near the O2 Arena in Greenwich.

The clip was filmed by a passenger on the Emirates Air Line but was considered a possible hoax after it allegedly first emerged on April Fool's Day.