There have been numerous reported sightings of the Loch Ness Monster through the years. Very few, if any, have been in the Thames, though.
The video was taken by Penn Plate as he was travelling on the Emirates Air Line, a cable car journey near Greenwich that offers views of the capital.
“This was on the cable car in Greenwich yesterday. Something huge was moving under the water and then briefly surfaced,” he said. “Are there whales in the Thames? Or is it some weird submarine?”
The video, which has been watched more than 500,000 times, drew more than 190 comments on YouTube.
“Lochness monster… confirmed!” one user commented. “Nessie!” another said.
Many speculated that it was a whale while some joked that it was another mythical creature – Godzilla.
One practical Twitter user played down speculation that it was the Loch Ness Monster, saying: “It swam all the way from Loch Lomond without being spotted did it, urm no I don’t think so.”
A variety of aquatic creatures, including dolphins, porpoises and seals, are spotted from time to time along the tidal River Thames because of the increased number of fish in the river, according to a survey by the Zoological Society of London.
In 2009, a humpback whale was found dead near Dartford Bridge and three years earlier a whale became stranded in the river before dying from dehydration.
The female northern bottlenose whale, which had drawn huge crowds along the river’s banks, was thought to have got lost on its way to feeding grounds in the Atlantic.