After British Divers Marine Life Rescue were alerted, they confirmed the sighting, noting that the whale seemed to be "swimming strongly".
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.
A beluga whale has been spotted in the Thames, between Gravesend in Kent and Tilbury in Essex.
Belugas live in estuaries, continental shelves and slopes, and deep ocean basins in open water, loose ice, and heavy pack ice.
Tanya Ferry, environment manager at the Port of London Authority, said it was unclear what the whale could eat.
Conservationists had been waiting to see if the "very lost" whale had made it out to sea after it was first spotting Tuesday afternoon in Kent.
He urged people to give the whale "space and minimise disturbance". "There have been just 20 sightings of beluga whales off the United Kingdom coast previously, but these have occurred off Northumberland, Northern Ireland and Scotland".
The RSPCA animal welfare group said it was "working with other agencies to monitor the situation".
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.
According to National Geographic magazine, beluga whales can range from 13 to 20 feet in length. They can be found in the waters around Russia, Alaska, Canada, West Greenland, and Svalbard.
The whales were nicknamed "canaries of the sea" by early whalers due to their squeaks and squawks.