Speaking on ITV2's NTAs: The Party last night, Anthony McPartlin revealed that the pair received a phone call while presenting I'm a Celebrity in November.
McPartlin said: "We were in Australia and we got the phone call basically saying, 'Bruce is opening the National Television Awards this year, and he wants to ask you two to do a duet with him'. We didn't even hesitate, it was a split-second yes!
"Because the thing is, he's a national treasure. He's performed with Morecambe and Wise and Sammy Davis Jnr. This might be the only chance we get to perform with Bruce Forsyth."
Watch Bruce Forsyth and Ant & Dec's surprise rendition of 'Let There Be Love' below:
Meanwhile, on winning their 11th successive gong for 'Entertainment Presenter', Declan Donnelly gushed: "Having done ten in a row, you kind of think, 'That's it - we probably won't get it this year'. Then you get here and get really excited."
However, McPartlin added: "It gets worse every year because there will come a point when we don't win it and everyone assumes we're gonna win it."
Regarding their prize for I'm a Celebrity... Get Me Out of Here, Donnelly said: "It was lovely to win for I'm a Celeb, because it was such a good year, and the cast were so good. We all felt it was really good.
"Shows in their eleventh series don't usually keep winning awards. All the guys behind us, Mark Wright, Dougie Poynter and Fatima Whitbread, were really nervous."
> National Television Awards winners list - In Full
> National Television Awards 2012 - Live Blog
Watch Ant & Dec speaking backstage at the NTAs below:
See a gallery of red carpet arrivals from the NTAs below: