"For anyone twitching the #BELUGA its been feeding around the barges (see last tweet for location) for the last hour and hasn't moved more than 200m in either direction".
Images posted on Twitter showed a white whale in the water.
A sighting of a whale on its own could indicate it is lost or disorientated. It said it is ready to provide help to the whale if asked to do so by other agencies.
Rob Lott, a marine mammal scientist at Whale and Dolphin Conservation, said the whale was being monitored in case it "live strands" on a sandbank.
An RSPCA spokesman told HuffPost UK: "The RSPCA is aware of reports of a whale - possibly a beluga - in the Thames".
A spokeswoman for the organisation said it was a "very rare occurrence", and urged people not to go out in boats to get a close look at the whale, but to watch it from the shore.
Despite the latest whale sighting prompting excitement on social media, conservationists expressed their worry for the animal.
"Beluga whales inhabit cold, arctic waters off Greenland, Svalbard and in the Barents Sea", the group said in a statement.
In January 2006, an 18ft northern bottlenose whale swam up the River Thames, with thousands of people watching from the riverbank as rescuers tried in vain to save it. "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".
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