"This is the most southerly sighting of a beluga we have ever seen around these shores", Lucy Babey, head of science and conservation at the ORCA charity, was quoted as saying by The Mirror newspaper.
Rescue teams are on standby in case the whale, which usually lives in the Arctic, gets into danger.
According to National Geographic magazine, beluga whales can range from 13 to 20 feet in length.
The sight of a beluga whale so far south - 1,000 miles (1,600 kilometres) from even Iceland - is exceptional.
It said it was ready to provide help to the whale if asked to do so by other agencies.
The RSPCA said it was aware of reports of a whale in the Thames.
He said he hoped the whale would find its way home safely, but warned the species also faces threats in its Arctic home, including shipping, oil and gas development and loss of sea ice habitat due to climate change.
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Benny, as it has now been named, appeared to be "swimming strongly" and feeding in the estuary but concerns for the whale's health have continued.
The last reported sighting of a beluga in United Kingdom waters was in 2015, when two were spotted off the northeast coast of England and one in Northern Ireland.
"He or she is obviously very lost and quite possibly in trouble".
"We would urge that the whale is given space and disturbance is kept to a minimum".
In 2006, a whale died after it swam up the river into central London despite efforts to rescue the animal. They can move between salt and fresh water. They can be found in the waters around Russia, Alaska, Canada, West Greenland, and Svalbard.
Beluga calls variously resemble a cork being prized from a bottle or a creaking door, along with sounds described as clicks, squeaks, chirps, bleats, moans, groans, and whistles.