Ecologist Dave Andrews first shared video of the mammal on social media yesterday, which he said was swimming in the river off Coalhouse Fort, Essex. It was spotted feeding near a number of barges.
Beluga whales, which are easily identifiable by their white markings, usually inhabit the cold Arctic waters off Greenland, Svalbard and the Barents Sea.
Rob Lott, a marine mammal scientist at Whale and Dolphin Conservation, said the whale was being monitored in case it "live strands" on a sandbank.
The British Divers Marine Life Rescue said the presence of the beluga is "concerning as it's not a common species, however it's swimming strongly and feeding", according to Sky News.
Tanya Ferry, environment manager at the Port of London Authority which is monitoring the whale, said it was unclear what the whale could eat.
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The WDC's Danny Groves told the UK Press Association, "He or she is obviously very lost and quite possibly in trouble".
"In the summer of 2015 two were spotted off the Northumberland coast and one in Northern Ireland", he said.
"We would urge that the whale is given space and disturbance is kept to a minimum", he said.
The beaked whale, dubbed the Prince of Whales by some, became a sensation in London with large crowds gathering to watch the ultimately fruitless rescue attempts and near 24-hour media coverage.
Known as the "canary of the sea" due to their chirps, clicks and whistles, beluga whales can range from 13 ft to 20 ft in length and have distinctive round foreheads, known as "melons".