A self-declared UFO expert believes he's come across the "Holy Grail" of evidence after allegedly spotting a giant "boomerang-shaped" aircraft on "an ISS livestream".

The video, which was shared online by conspiracy theorist 'Disclose Screen The Grimreefar', shows the V-shaped object flying above the Earth, claiming it came from a livestream from the International Space Station.

The self-professed extraterrestrial expert shared the footage to his YouTube channel, where he described it as the "Holy Grail" of alleged UFO evidence. He explained to his 14,000 subscribers that he had held onto the clip for eight months as he had it checked by a friend for any "signs of editing."

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The V-shaped object flying over the Earth has been described as a 'boomerang UFO'
The V-shaped object flying over the Earth has been described as a 'boomerang UFO'

He claims his pal found "no evidence" that the footage had been manipulated. In the video, which has gained more than 700 views, he continued: "For me, this clip is the holy grail of UFO sightings. This is definitely not man-made.

"The sheer size of it would suggest otherwise. It's hard to tell the distance from the camera, but it's close enough to the earth that you can see it casting a shadow.

"The ISS orbits the planet at around 250 miles from the Earth's surface. So if this UFO is closer to the Earth than it is the camera, then we are looking at something that is miles in diameter.

"This isn't a scouting ship. This is a colony ship. There is an entire population on this thing. This craft fits the description of the Phoenix lights perfectly. POW!"

Alien looking through the window of a spaceship in earth orbit, illustration
The self-professed ET expert believes the clip was the 'Holy Grail' of UFO footage (stock image)

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Viewers were left fascinated by the clip as many took to the comments believing it could be proof of extraterrestrial life. One user said: "Holy S**t! If it is real, then POW! Top catch right there man."

Another added: "That thing is massive," while a third commented: "That UFO footage is incredible. There has been a lot of great UFO video lately of UFOS above the clouds lately. Great video."

Meanwhile, others took a little more convincing, as a viewer determined it was "100% FAKE." The continued: "The boomerang's framerate is 6 times higher than the Earth beneath..... when you show in slowed down you see the boomerang move 6 times and THEN the background of the Earth moves once to the right.... 6 of boomerang, 1 of Earth.... and it goes on till it's out of frame..... this is IMPOSSIBLE if it was filmed with a camera... unless the Earth suddenly started moving in slow ticks lol."