The RSPCA says a beluga whale that's swimming in the Thames near Gravesend may stay in the river for a few days as long as there is a food source.

But whale watchers are now reporting that it may be heading down the Thames towards London.

The mammal, which is normally found in the Arctic, was first spotted on Tuesday.

Conservationists are monitoring the creature amid fears that it could become distressed, but so far Benny, as he has been nick-named, is said to be swimming strongly and feeding well.

The Gravesend town centre manager twitter account posted: "If you're new to Gravesend today #WhaleWatching you're very welcome.

"I met some folk yesterday from Thetford & Milton Keynes who travelled down to see the #Beluga #ThamesWhale."

RSPCA inspector Clare Dew said porpoises had also recently been seen in the Thames and the beluga may have arrived with them.

Although belugas are social animals and usually travel in a pod with other whales, they sometimes attach themselves to schools of dolphins or porpoises.

She said: "There have been a lot of hurricanes and typhoons in some parts of the world recently which have disrupted these animals' normal food source.

"It's likely the beluga just followed the food source south to the Thames."

She said Benny would be left alone to make up his own mind when to return to the ocean.

She said: "There is plenty of food for it here and no big predators."

Inspector Dew said the only real risk to Benny came from the container ships and other vessels moving about the Thames.

Benny is not likely to swim further up river, because he prefers salt water.

It is believed to be the farthest south a beluga has ever been spotted.