Nail-biting footage of two thrill-seekers climbing to the top of One Canada Square in London's Canary Wharf has emerged online.
In the 10-and-a-half minute clip posted on YouTube, the two young men enter the second tallest building in the UK at night, before climbing over security barriers and then using the lifts and stairs to reach the top.
Arriving at the 50th storey the pair discover that the doors that lead to the roof are unlocked, and climb out onto the roof of the 770ft (235m) tower.
Once on the roof the pair decide to climb to the top of the building's iconic flashing pyramid so they can touch the top.
One of the pair is believed to be 19-year-old Harry Gallagher, who regularly posts videos on his YouTube channel - which has more than 510,000 subscribers - of himself climbing to the top of skyscrapers.
Also in the video is his friend who is referred to as "Dibs" and who's Instagram contains numerous pictures of the pair in unlikely locations.
Speaking to The Times, Mr Gallagher's mother, Amanda, said she was “extraordinarily proud” of her daredevil son.
However, she did concede her son's antics were "a double-edge sword".
Adding: “Nobody wants their child risking life and limb.
"His [older] brother was the freerunner in the family but he gave it up because decided he didn’t want to kill himself. Harry never had that fear.
"He just has this passion for it. He is fearless.
“As a parent, you can’t stop that journey. You can’t repress them. You have to find the strength to just roll with it.”
The Metropolitan Police said they are aware of the footage which "depicts people gaining unauthorised access to One Canada Square".
Adding: "We will liaise with the building operators to assess whether any offences have been committed.
"Enquiries continue and no arrests have been made."