Astonishing footage circulating online shows a beluga whale swimming in the River Thames.

Consultant ecologist and ornithologist, Dave Andrews, claims he spotted the beluga whale, and filmed the footage around 11am yesterday.

The video was taken between Tilbury, Essex, and Gravesend, Kent and shows the whale swimming above water before diving deep under as it hunted food around the barges.

Dave tweeted as he was watching the whale swim for around four hours.

He wrote on Twitter: "Can't believe I'm writing this, no joke - BELUGA in the Thames off Coalhouse Fort.

"For anyone twitching the #BELUGA its been feeding around the barges (see last tweet for location) for the last hour and hasn't moved more than 200m in either direction. Still present."

Two hours after his original tweet, Dave added: "Still here and showing well #BELUGA."

The beluga whale or white whale are commonly found in the Arctic Ocean and have a stocky body. They often feed on fish and crustaceans.

Sightings of Beluga whales are extremely rare on the coasts of the UK and normally only occur off the coast of Scotland.

But marine life experts have expressed concerns for the whale's health, urging members of the public to stay away from it.

They confirmed it was "almost certainly" a Beluga whale and urged the public "to watch it from the shore".

The Port of London authority confirmed it was aware of the video.

The RSPCA said it was sending an officer to look into the matter.