The mysterious shape was spotted in the river by Westminster, Vauxhall and Hammersmith by witnesses who shared photos of it on social media. In one shot, what looks like the shadowy outline of an animal can be seen underneath the water.

Posting a picture of the fin by the London Eye, Twitter user Yasmin Dan wrote: ‘Walked through central today and looked in the river… is that a shark in the Thames??!’

Another Twitter user by the name ‘Cam’ then replied to say they were ‘100% sure’ they had seen a moving fin close to Hammersmith Bridge.

A spokesperson for the Port of London authority was unable to confirm what the object was, stating that it could be a sea creature, but was unlikely to be a shark.

In a statement, they said: ‘The photos we have seen are not very clear. The dark object could well be a piece of debris or driftwood.

‘None of our teams have reported an animal in the river today. However we cannot give a definitive view either way from that photo. If anyone spots anything similar do please let us know!

‘A shark is however very unlikely – though species like the dog fish and smooth hound have been seen further out in the Thames Estuary.

‘However, if it is the dorsal fin of an animal, it is more likely to be that of a small cetacean such as a harbour porpoise. These are not uncommon in the Thames and have been spotted in central London on many occasions.’

In 2006 a five-metre long northern bottlenose whale was discovered in the Thames. A rescue operation was then launched after the creature was beached several times due to the shallow depths of the river.

Sadly the whale, a young female, did not survive the rescue and it was later revealed she was suffering from dehydration, muscle damage and kidney failure.

Last year two whales were both found dead in the water, two weeks apart. The first whale, a humpback nicknamed Hessy, died after being hit by a ship.

The second whale then washed up near Gravesend, less than five miles from where Hessy had been pulled from the water.