As Willoughby was about to record the results show to go out on Sunday night (May 13), she overheard her bosses watching the Talent result through her earpiece, The Sun reports.
Willoughby allegedly heard Ant and Dec's voices while she was asking Sir Tom Jones to tell her which acts he was going to send home.
An audience member revealed that she said: "Gallery, I can't believe you've got that on up there!"
"People in the audience fell about laughing," the source added. "Even The Voice bosses wanted to know who had won Britain's Got Talent."
The Britain's Got Talent final became the most watched programme of 2012 so far, easily beating The Voice UK in the ratings on Saturday (May 12).
The Voice UK said goodbye to four more contestants on Sunday, with four more to leave next weekend.
The Voice UK continues next Saturday (May 19) at 6.30pm on BBC One, when Jessie J and Danny O'Donoghue's teams battle it out to stay in the competition.