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Rescue teams are on standby to ensure than a Beluga Whale spotted in the River Thames does not become stranded.
The whale was spotted moving through the water near Gravesend , Kent on Tuesday afternoon.
It was spotted by Dave Andrews, a consultant ecologist who wrote on Twitter: “Can't believe I'm writing this, no joke - beluga in the Thames off Coalhouse Fort.”
Rescue teams are monitoring the animal's location to ensure its safety.
Robb Lott, a marine mammal scientist at Whale and Dophin Conservation told BBC Radio 5Live: “It's a monitoring operation at the moment, obviously the longer it stays in the Thames estuary then it will become more of a concern, so we'll see what happens in the morning.”
He added that to see a Belgua whale by itself is “concerning.”
Beluga whales can grow up to 20 feet in length and are usually found in the icy waters around Greenland, Svalbard or the Barents Sea.
They are said to be social animals.
Members of the public have been urged to keep their distance.
The British Divers Marine Life rescue is sending an area coordinator down to the river to monitor the situation.
The network of volunteer "marine mammal medics" has whale rescue pontoons at the ready that can be used to re-float stranded animals in an effort to move them to safety.
A beluga whale breeches near a buoy on the River Thames near GravesendToby Melville/ReutersThe last reported sighting of Beluga in UK waters was in 2015, when two were spotted off the Northumberland coast and one in Northern Ireland.
In 2006, a whale died after it swam up the river into central London despite efforts to rescue the animal.
Additional reporting by Press Association.