Londoners would be forgiven for thinking their rivers were lacking in underwater life, owing to traffic and pollution on the city's waterways.

Officers at the Marine Policing Unit have blown that popular assumption out of the water, however, after they released a video showing a lone dolphin swimming up the river Thames.

In a tweet they said: "Taking care not to get too close, Marine 2 got a tantalising glimpse of the Thames #Dolphin at Wandsworth this morning. #London #wildlife"

Taken on Sunday (November 5) the video shows the tail fin of a dolphin bobbing above the water. Police say the video was shot in the riverside borough of Wandsworth, around 25 kilometres from the mouth of the river Thames. This means the dolphin would have swum past popular landmarks such as Tower Bridge and Big Ben to get to its location.

They confirmed that the animal was indeed a dolphin and not a porpoise, which are more commonly seen in rivers.

The dolphin was also curious to get a look at officers despite being in such an unfamiliar location.

"Despite our best efforts to keep clear it would seem #Dolphins are as interested in us as we are of them. Popped up next to us (engines off)."

Officers also claimed that sightings of other sea life, such as seals and porpoises, can be seen from their boats occasionally.

"It's not as unusual as you'd think. There are regular sightings of porpoises, seals and dolphins up and down the Thames", they said.