A two-and-a-half minute video taken from the banks of Loch Ness appears to show a large creature moving through the water.
Experts described it as the most compelling footage to emerge in the past 20 years of hunting for the Loch Ness Monster.
The clip shows what look to be humps breaking the surface of the water and something powerful swimming and turning underneath.
A strong wake of water can be seen behind the unknown creature as it swims along just below the surface, and just 150 yards from shore.
Gary Campbell, who has spent 26 years logging every credible sighting of Nessie, said it was the best footage he had seen in decades.
This famous photo said to be the head and neck of Nessie was taken in 1934 by holidaying London surgeon, Robert K. Wilson, in Loch Ness. It was later judged to be a hoax
He added: ‘This is a really, really good sighting of something that is unexplained in Loch Ness.’
The remarkable footage was taken by a couple who are professionals in their early 50s, who were on holiday in the Scottish Highlands.
They said they had no strong views on Nessie other than having been amused by anecdotes about previous sightings.
The husband and wife, who want to remain anonymous, are keen for their footage to be analysed in the hope it may help provide definitive answers about the existence of the monster or what might be inhabiting the loch.
The woman said: ‘We were fairly ambivalent about the Loch Ness monster. We enjoyed the stories but didn’t necessarily believe them.
‘We’d definitely love for somebody to analyse the video. If it helps anybody work out what’s down there then it’d be our absolute pleasure. We would be delighted.’
Another mysterious sighting linked to the alleged monster captured in Loch Ness 2002
The woman had woken up just after 6am on Monday and decided to take some photographs as the loch was so still that its surface was like glass.
But after spotting something large moving in the water, she began filming on her iPhone and could be heard saying ‘What on Earth is that?’ as her husband joined her to watch what was unfolding.
The couple filmed for two minutes and 37 seconds from 6.21am as the mysterious creature – they estimated it to be between 20 and 30 feet in length – swam beneath the water and gave them tantalising glimpses as it broke the surface.
It appeared to have at least one fin or limb, which paddled under the water ‘like an oar’.
The woman also took several still pictures from the vantage point of their holiday cottage set on the hillside opposite Urquhart Castle, around a quarter of a mile from the loch.
She added: ‘I really don’t know what it was in the water. It was something large. I don’t think it really equates on the video quite how large it was.
‘We don’t know what the creature was. It was propelling itself with something. It wasn’t how a fish would do it.
‘You could see it much clearer than it’s come out in the photos. The lumps or humps or whatever they are kept disappearing under the water, but it was still pushing forward under the water.
‘You could see something under the water, then it rushed forward and turned round. From what we could gauge, it was between 20 and 30 feet long. There was nothing else on the loch, no boats, nothing.’
Her husband said: ‘We were not here to look for the Loch Ness monster, we wanted to enjoy the beautiful scenery.
‘There is something weird went on out there on the loch. Somebody far more intelligent than us on a scientific basis can analyse it and if it helps contribute anything to the understanding of the phenomena or even if it rekindles a bit of interest and fun in a world that’s full of so much misery at the moment then that can’t not be a good thing.’
The footage has now been handed to Mr Campbell, who keeps the Official Loch Ness Monster Register and only includes sightings which cannot be easily explained away. He has now officially logged the couple’s sighting.
Mr Campbell said: ‘In terms of video evidence, there’s been two or three really good videos in the past but this is certainly up there with the best of them.
‘When something like this comes along which is genuinely inexplicable then it is great.’
Photo of a mystery object in Loch Ness taken by London student Jolene Lin in 2016. She described it as looking like a large serpent and is convinced that it was the Loch Ness Monster
Mr Campbell described the footage as the best taken since August 2000 when Glasgow postman Bobbie Pollock shot a three-and-a-half minute video of an unidentified creature in the loch.
But Dr Darren Naish, a zoologist and author of Hunting Monsters, said he was doubtful a monster would ever be found in Loch Ness.
Describing the new footage, he said: ‘We see two tiny black objects, filmed at great distance.
‘They’re nothing more than unidentifiable specks on the water and there’s no indication that we’re seeing something that’s especially large or unusual in shape.
‘Based on how boats look from the same approximate location and distance, the objects must be small – I estimate less than 50cm long.
‘I see no reason why they can’t be birds – probably ducks. The wakes they leave on the surface look interesting, but wakes of this sort are common on the loch’s surface when conditions are right.’