Excitement gripped the river community in Putney after a porpoise was spotted near the embankment.
The porpoise, whose Latin name is phocoena phocoena, was seen by eagle-eyed members of the public at various points along the river in Putney last week.
Initial reports suggested it was a dolphin but the Environment Agency have now confirmed it was a harbour porpoise.
Harbour porpoises are the smallest and most common cetaceans (whales, dolphins and porpoises) in European waters.
They are also one of the shortest lived, rarely surviving beyond 12 years of age.
Harbour porpoises favour the shallow waters of coasts and estuaries and it is not uncommon for them to travel a considerable distance up river.
The photo was taken by Toby Forester who uploaded the image on to Twitter.
The mammals have been spotted in the River Thames before, such as in 2011, when RNLI volunteers pulled the dead body of a porpoise out the river in Chiswick.
In 2006 people were captivated by the plight of a whale that swam up the River Thames and died despite an attempted rescue.
Under UK law, whales, sturgeons and porpoises are royal fish, which means they are classed as the personal property of the monarch.
The law states that if species are discovered dead or in distress the Receiver of Wrecks must be notified along with the Natural History Museum.
Comments: Our rules
We want our comments to be a lively and valuable part of our community - a place where readers can debate and engage with the most important local issues. The ability to comment on our stories is a privilege, not a right, however, and that privilege may be withdrawn if it is abused or misused.
Please report any comments that break our rules.
Read the rules hereComments are closed on this article