A beluga whale has been spotted in the Thames near Gravesend.
Twitter user Dave Andrews, a consultant ecologist and ornithologist according to his Twitter biography, posted a video of the whale saying: "Can't believe I'm writing this, no joke - BELUGA in the Thames off Coalhouse Fort."
He added the beluga had been feeding around barges on the river for around an hour and had not moved more than 200m either direction on the river.
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.
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.
A spokesman for the RSPCA said: "The RSPCA is aware of reports of a whale, possibly a beluga, in the Thames.
"We are working with other agencies to monitor the situation and ready to provide appropriate assistance if requested."
While the report of the animal prompted excitement on social media, conservationists warned that the beluga whale, normally found in the High Arctic, was lost and could be in trouble.
Danny Groves, from Whale and Dolphin Conservation (WDC) said: "This is a High Arctic species thousands of miles from where it should be in Greenland, Svalbard or the Barents Sea, they are usually associated close to the ice.
"He or she is obviously very lost and quite possibly in trouble. "
He urged people to give the whale "space and minimise disturbance".
Mr Groves said it was not the first time a beluga has been spotted in UK waters in recent years.
"In the summer of 2015 two were spotted off the Northumberland coast and one in Northern Ireland," he said.
In 2006, a whale died after it swum up the Thames into central London despite efforts to rescue the animal.
After being monitored for two days, and making it all the way to central London, the whale died during a rescue attempt.