A 'fearless' tentacled beast was spotted swimming in the River Thames one year before terrifying a young boy.
Glancing into the water from London Bridge, two brothers were stunned by a marine creature that does not match any known existing species.
The closest animal they could compare it to was a kind of squid and jellyfish hybrid - neither of which are common in UK rivers, cryptozoology enthusiast Andy McGrath reports.
The Beasts of Britain told Daily Star Online: "The brothers described the creature as having a greyish, featureless and slightly elongated square head, with what looked like lots of green tentacles or tassels at the bottom.
"It was around 5ft in length and as thick as a man’s body. The creature was sighted to the left of the bridge, on the surface of the water and was swimming slowly against the current.
"After observing the animal submerging and resurfacing, for around one minute, they finally lost sight of it."
They added that the creature had a metallic or mechanical feel to it, but insisted it was alive when they saw it in central London in April 2014.
The brothers claim to have attempted to capture the creature on camera, using their phones but the quality and distance proved too poor.
A debate broke out on the bridge between other witnesses, as to what the creature might be.
Brother Loz believes the creature had barely moved a year later when he returned - this time beneath the bridge at low tide - and a sense of panic overwhelmed his son.
Andy said: "His son, ordinarily a very calm and confident boy, suddenly became hysterical and demanded to leave the area, claiming that he could feel something in the water watching them."
Loz said that his boy had never heard of their encounter 12 months earlier which could have played on his mind.
In search of a more plausible explanation than a tentacled mutant as described, Andy found just two examples of evidence showing similar animals on Britain's shores.
He said: "This sighting is very unusual and somewhat unique, something like a creature resembling both a squid and a jellyfish. It certainly does not match any currently known Medusozoan or Cephalopod species known to science.
"Although, to the uninitiated a box jellyfish may seem to possess some of these physical characteristics.
"I found photographic evidence of a giant Bell Jellyfish, 4ft across, which was sighted in a Cornish river, in May of 2014, and know that many of these types of jellyfish, although not typically sighted in rivers, are frequently sighted along the coasts of Britain.
"But the Bell Jellyfish has an orange-pink hue and does not match the description of the London Bridge creature.
"In 1925, a giant squid washed up on the banks of the Humber in Withernsea Beach, but again, I could find no species of squid, either small or large, that matched the description of the London Bridge creature."
Andy warned: "So in future, whether you are crossing London Bridge on your daily commute or searching for Saxon treasures in the mud! Keep a wary eye on the water, for something weird and wonderful might just be waiting there, for you."