The TV duo, who won 'Best Entertainment Presenter' for the 11th year in a row last night, described Cowell as "the best talent show judge in the world" and claimed that he is making the acts and judges work harder than ever before to make the show a hit.
"Simon is back and don't tell him, but he's probably the best talent show judge in the world," said Dec.
"He's upped the stakes this year. He's not giving an inch. He's given us all a tough time. It's been the longest auditions we've ever had.
"He's making singers perform again and again, he's making them sing three songs. He wants to find real stars. He's determined to find a global star.
"I think it's going to be a good year. We've only done two days in Manchester and one day in Blackpool, but it's already shaping up to be a really good year."
Speaking about the new judges David Walliams and Alesha Dixon, Dec added: "They have hit the ground running on day one... David is brilliant, he's very mischievous, he's winding Simon up a treat. And Alesha is great, she's feisty, she knows what she's talking about."
Amanda Holden is currently missing Britain's Got Talent auditions following the birth of her daughter Hollie Rose Hughes.
> Doctor Who, Coronation Street lead National Television Awards
> National Television Awards winners list - In full
> National Television Awards 2012 - Live Blog
> National Television Awards 2012: The Winners - in pictures
> National Television Awards red carpet arrivals - in pictures