The mischievous duo, who were reunited this year after McPartlin’s hiatus, joined forces with Britain’s Got More Talent host Stephen Mulhern as part of a DJ act while dressed as dogs.
A show source told The Sun that it was “one of the moments of the day” and that they even had their dog costumes made for the elaborate trick, which was thought to be the last of the series but still captured by the cameras.
“Everyone was spent having watched loads of auditions, but Ant and Dec thought it would be a good way to finish things off,” the insider said. “[The] DJ set which went down a treat with the crowd despite no one having a clue who they were.
“The judges were in stitches when they stripped off at the end.”
It is unknown if the prank will be broadcast at any point during the series, including the main show and ITV2 spin-off.
Kicking off at the beginning of April, series 13 saw McPartlin and Donnelly reunited after Donnelly was left flying solo for the live show week last June.
McPartlin decided in March 2018 to step down from his TV commitments to focus on his recovery from addiction during a tumultuous period, which also saw him plead guilty to drink-driving.
He said: “It was lovely to see when they stepped back on the stage together in London. How excited people were to see them together.
“It was really emotional and I was pleased for them. It was just how it always was.”
Britain’s Got Talent continues Saturdays on ITV with Britain’s Got More Talent airing straight after on ITV2.