Waking up at 6am, the couple discovered the man, who had just scaled the enormous building. Paul Curphey, 52, with partner Treasaidh, were visiting the iconic London landmark for a birthday trip when the unexpected guest appeared.
The surprised sightseers noticed someone “waving at the window”.
Free-climber Adam Lockwood had earlier shared a selfie on Facebook which captured him standing terrifyingly close to the edge of the huge building. The Shard stands at a whopping 310m (1,016ft) tall.
It’s not the first wild escapade for Mr Lockwood. He has also scaled Dubai’s second-tallest building, Marina 101, which reaches an eye-watering 425m (1394ft).
Mr Curphey, a retail businessman from the Isle of Man, snapped a picture of Mr Lockwood. He said he was “yelling in a celebratory fashion” as he dangled from the side of the building.
Mr Curphey added: “He appeared, waving at the window, 40 floors up, already in the deep end so to speak. We couldn’t help but urge him on to complete his mission.
“He was smiling, waving, and having the time of his life.”
Mr Curphey added that his partner initially thought he had arranged an extreme birthday present.
He joked: “(My) partner thought I had pulled all the stops out and managed to get a guy to bring a box of Milk Tray for her birthday.”
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The incredulous businessman added: “It was scary to see, but his happy demeanour was amazingly uplifting.”
Following the incident, the Metropolitan Police said a 21-year-old man has been arrested on suspicion of trespass. Two other men were arrested on suspicion of causing public nuisance during the incident.
The force arrived at the scene, close to London Bridge, around 5.38am on Sunday morning. They were joined by the London Ambulance Service and London Fire Brigade.