Conservationists have reported a fresh sighting of a "very lost" beluga whale in the Thames as they wait to see if it will head out to sea.
The animal welfare group says it is ready to provide help to the whale if asked to do so by other agencies.
Rescue teams are on standby in case the whale gets into danger.
Among the theories of how the beluga whale ended up in the Thames is that it followed a shoal of fish into the waterway.
The sight of a beluga whale so far south - 1,000 miles (1,600 kilometres) from even Iceland - is exceptional.
Beluga whales (Delphinapterus leucas) normally live in Arctic and sub-Arctic waters, so any beluga in the Thames would be a long way from home.
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The Whale and Dolphin Conservation society said the whale could be ill, or young and separated from his or her pod.
Beluga whales were last spotted in the United Kingdom three years ago off the coast of Northumberland and Northern Ireland, but sightings were "extremely rare", spokeswoman Julia Cable said.
He urged people to give the whale "space and minimise disturbance".
In 2006, a whale died after it swum up the Thames into central London despite efforts to rescue the animal.
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. They can move between salt and fresh water.
They are common to many regions of Alaska, as well as Russia, Canada, and Greenland.
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.