Ecologist Dave Andrews of the sea mammal on social media, which he said was swimming in the river off Coalhouse Fort, Essex.
He said: “Can‘t believe I‘m writing this, no joke – BELUGA in the Thames off Coalhouse Fort.”
Image: The beluga whale was spotted off Coalhouse Fort in Essex. Pic: Rob Powell/LNP
Mr Andrews added that the beluga had been feeding around barges on the river for around an hour and had not moved more than 200m in either direction on the river.
Beluga whales, also called white whales, are typically spotted off the coast of Scotland, as this is closest to their native waters.
For anyone twitching the 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. Heres another video
— Dave Andrews ()
A sighting of a whale on its own could indicate it is lost or disorientated.
Danny Groves, from Whale and Dolphin Conservation (WDC) said: “This is a High Arctic species thousands of miles from where it should be in Greenland, Svalbard or the Barents Sea, they are usually associated close to the ice.
“He or she is obviously very lost and quite possibly in trouble.”
Mr Groves urged people to give the whale “space and minimise disturbance”.
The mammals can live up to 30 years in the wild and usually travel in groups or “pods”.
“We are working with other agencies to monitor the situation and ready to provide appropriate assistance if requested.”
Image: Photographers on the banks of the Thames
The British Divers Marine Life Rescue, which helped with the rescues of stranded cetaceans and other marine animals, said they were sending their area coordinator to monitor the situation.
A spokeswoman for the organisation said it was a “very rare occurrence”, and urged people not to go out in boats to get a close look at the whale, but to watch it from the shore.
In 2006, a northern bottle-nosed whale up to 18ft-long (5m) became stranded in the Thames for two days.
Image: In 2006, a northern bottle-nosed whale died after it swum up the Thames
She died from convulsions as she was being rescued.
Despite the latest whale sighting prompting excitement on social media, conservationists expressed their worry for the animal.
Can't believe I'm writing this, no joke – BELUGA in the Thames off Coalhouse Fort
— Dave Andrews ()
It is not the first time a beluga has been spotted in UK waters in recent years.
“In the summer of 2015, two were spotted off the Northumberland coast and one in Northern Ireland,” Mr Groves added.