A beluga whale was spotted on Tuesday near barges in the River Thames near London, hundreds of miles from its normal Arctic habitat, naturalists said.

"Can't believe I'm writing this, no joke - BELUGA in the Thames off Coalhouse Fort," ecologist Dave Andrews wrote.

Video footage showed the white mammal surfacing in the Thames near Gravesend, Kent.

"This is the most southerly sighting of a beluga we have ever seen around these shores," Lucy Babey, head of science and conservation at the ORCA charity, was quoted as saying by The Mirror newspaper.

The last sighting in UK waters was in 2015 when they were spotted off northeastern England near the Northumberland coastline, but they left shortly afterwards, she added.

In 2006, a northern bottle-nosed whale died after stranding itself in the Thames.