Watch: Footage shows dolphin swimming down River Thames

A passerby was running along the riverside before stopping off at The Angel Pub, in Bermondsey, where they noticed the animal at around 7.40pm Wednesday evening (July 31).

He said: “My friend turned to me and said ‘I can see a fin'.

“It was quite exciting - everyone was looking over at the river and everyone was in pure shock."

Sightings of the mammals are on the up, though experts say it's a "red flag" for them to be spotted this far inland.

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Mary Tester, from British Divers Marine Life Rescue Team, said: “Obviously we love them very much, but when we see them in the Thames it is a red flag.

“Porpoises are fine, seals are fine, but dolphins when they come this far up the river, is a warning sign.

“So if you do see a dolphin in London it’s really important to call us so we can come out and monitor the mammal, and see if there’s anything further we can do to help it.

“We have seen so far this year five pods of dolphins coming up beyond the estuary and into the Thames. Three of those have come into London. All three of those have not survived, unfortunately.”