Top wildlife experts today admitted they are stumped by footage apparently showing a “strange creature” lurking in the Thames.
Marine biologists and conservationists said they had no answer as to what the long dark shape spotted in the river next to The O2 could be.
A video, taken from an Emirates Air Line cable car, shows a huge object with what appears to be two humps emerging from the water before slipping back beneath the surface.
The grainy footage has fuelled speculation of a whale or submarine sighting.
Others joked it could be the Loch Ness Monster on a spring break from the Scottish Highlands.
Ian Tokelove, from the London Wildlife Trust, said despite intense scrutiny the mystery remained unsolved.
He told the Standard: “We are not aware of anything that large and moving in the Thames.
“We had a good look at the footage but it isn’t clear enough to make out what we are looking at.
“We would encourage everyone to go down to the river because there is a lot of exciting wildlife around.
“There are more than 100 species of fish and seals are not uncommon.”
Jamie Oliver, senior curator at Sea Life London Aquarium, added: "Given that size I have no idea what that could be if it is an animal."
Experts from the Marine Conservation Society and Natural History Museum declined to comment because they said the footage was too unclear.
YouTube user Penn Plate, who took the video, wrote: “Something huge was moving under the water and then briefly surfaced.
“Are there whales in the Thames?? Or is it some weird submarine?”
Vermin seen around London
Last year, the Standard reported that more than 2,000 seals and 450 porpoises and dolphins have been spotted in the Thames over a decade.
The Zoological Society of London (ZSL) said Canary Wharf is the most likely area to spot large incongruous marine life because of its high vantage points.
Seals had been seen as far upstream as Hampton Court, with dolphins and porpoises sighted at Teddington Lock in south-west London.
