"We are surprised and a little saddened that it has resurfaced again - we had hoped it had gone out to deeper waters and gone out to where it should be", said Lucy Babey, head of science and conservation at the ORCA charity.
According to National Geographic magazine, beluga whales can range from 13 to 20 feet in length.
The rare marine mammal was first seen near Gravesend, Kent, and appeared to be "swimming strongly" and feeding in the estuary.
Beluga whales, which are easily identifiable by their white markings, usually inhabit the cold Arctic waters off Greenland, Svalbard and the Barents Sea.
Rescue teams are on standby in case the whale gets into danger.
Dave Andrews, a consultant ecologist, posted a video on Twitter at noon on 25 September, saying, 'Can't believe I'm writing this, no joke - beluga in the Thames off Coalhouse Fort.More news: Samsung Galaxy S10 To Welcome A Fourth Model?
"The Thames beluga is back feeding in its favoured spot around the barges on the Kent side", he said urging all boats to "keep clear and let it alone".
The last sighting in United Kingdom waters was in 2015 when they were spotted off northeastern England near the Northumberland coastline, but they left shortly afterwards, she added.
The Whale and Dolphin Conservation society said the whale could be ill, or young and separated from his or her pod.
In 2006, a whale died after it swum up the Thames into central London despite efforts to rescue the animal.
"We are working with other agencies to monitor the situation and ready to provide appropriate assistance if requested", it said. They have a rounded forehead and no dorsal fin.
Belugas are able to produce sounds such as chirps, clicks, whistles and squeals, giving them the nickname "the canary of the sea".