The Britain's Got Talent Champions winner has reportedly let slip their victory - a month before the show is due to air.
A new All Stars version of BGT will broadcast later this year on ITV.
Filming took place on last month (July) at Wembley Arena with the pre-recorded results determined by a mixture of the judges and a studio audience vote.
An air date for the series has yet to be confirmed but it's expected to start at the end of August.
However the winner is said to have given the result away after posing with their trophy outside the studio and taking selfies with fans.
An insider told the Daily Mirror newspaper: "Producers are fuming about the result being given away - the show doesn't even have an air date yet."
Another source told the Daily Mail about the leak: "I don’t think it was disrespect from the winners – in fact completely the opposite.
"It was over-excitement after picking up the trophy in front of an audience [at] Wembley Arena!"
Britain's Got Talent: The Champions will welcome back acts from both past series of BGT and Got Talent shows across the world.
Contestants in the all stars line up are set to include Series 1 champ Paul Potts as well as Stavros Flatley and 2012 Britain's Got Talent winner Ashleigh Butler, who won as dog act Ashleigh and Pudsey. Following the pooch’s sad death two years ago, she now performs with new dog, Sully.
More returning acts are said to include 2008 BGT winner George Sampson, 2011 champion Jai McDowall, 2014 winners Collabro, dance duo Twist and Pulse and singer Connie Talbot, who first appeared on BGT's first ever series when she was just 6.
International acts are rumoured to include American comedian Vicki Barbolak, ventriloquist Paul Zerdin, Ukrainian sand performance artist Kseniya Simonova and Neighbours star Bonnie Anderson, who won the first series of Australia's Got Talent in 2007.
Simon Cowell, Amanda Holden, Alesha Dixon and David Walliams will be judges on the series just like the main show.
Ant and Dec will host, which will air on ITV.