The beluga was first sighted on Tuesday near the southern Kent side of the river, downstream of Gravesend.
Rescue teams were on standby in case the animal, which is normally found thousands of miles away in the Arctic, gets into danger, and have urged people to keep their distance.
"The longer it stays in the Thames estuary then it will become more of a concern", he told BBC radio.
"Beluga whales are a species of the icy Arctic - finding one in the tepid Thames is an astonishingly rare event", said Rod Downie, polar chief adviser at WWF, the World Wide Fund for Nature.
They are also a social species, so to see one by itself in the Thames is "concerning", Lott said.
Belugas live in estuaries, continental shelves and slopes, and deep ocean basins in open water, loose ice, and heavy pack ice.
The British Divers Marine Life Rescue, which helps with rescues of stranded cetaceans and other marine animals, said they were sending their area coordinator down to the river to monitor the situation.
The whale spotted in the Thames was clearly a mature beluga whale, and possibly a female or a younger whale due to its size, Babey said.
"Can't believe I'm writing this, no joke - BELUGA in the Thames off Coalhouse Fort", ecologist Dave Andrews wrote on Twitter. "There have only been around 20 sightings of beluga whales off the United Kingdom coast previously, but these have occurred off Northumberland, Northern Ireland and Scotland".
Among the theories of how the beluga whale ended up in the Thames is that it followed a shoal of fish into the waterway.
"We do have quite a lot of plastic bags, which could be quite an issue", she said.