A pod of five harbour porpoise has been sighted in the River Thames.

At around 9.30am, the Marine Policing Unit was alerted to the sighting of a dolphin near Tower Bridge. They reported no further sightings until 10.40am, when they confirmed their boat Marine 2 was “following a pod of about five harbour porpoises in the Lambeth area of the river.”

Breaking #dolphin news.. Marine 2 are following a pod of about 5 harbour porpoises in the Lambeth area of the river. — MPSonthewater (@MPSonthewater) December 6, 2013

#dolphin update. The porpoises seem happy enough. They are surfacing regularly and are staying in the Westminster area. No good pics yet. — MPSonthewater (@MPSonthewater) December 6, 2013

Stephen Mowat, a marine conservationist for ZSL said it was uncommon to see so many so far up the Thames.

“With the condition of the Thames improving we are getting more and more sightings of marine mammals such as porpoise and seals,” he said.

Mowat said one of the reasons that the porpoise may be so far up the river was because of the storm surge.

“They follow prey fish up the river so with the high tides to get more fish further up the Thames. Because central London is a more heavily populated area more sightings will be reported.”

He said it would not pose a threat to the porpoise that the Thames barrier was closed. “They should be fine, they can find their way up river and go back down again. As soon as they’ve had their fill of fish they will find their way back out.”

And there it is! The #christmasdolphin captured by Marine 2 this morning. Pic taken from a video of it breaching. pic.twitter.com/RjQPDoB8LC — MPSonthewater (@MPSonthewater) December 6, 2013

In the last 30 years, the Thames has become one of the world's most unpolluted metropolitan tideways after being declared biologically dead in the 1950s as a result of heavy pollution. ZSL's research has confirmed that dolphins, porpoises and seals are all regular visitors to the Thames, with sightings recorded at the London Eye and Canary Wharf, and as far down as Richmond.

Earlier this year more than 700 seals were spotted in the Thames Estuary in the first ever count carried out by air, land and water.

Members of the public can report sightings on the Thames marine mammal survey form.