(Cover) - EN Showbiz - Simon Cowell admits that he "wasn't 100 percent sure" if he secured Britney Spears and Demi Lovato as judges.

The superstar songstresses officially announced at Fox Upfronts in New York City Monday that they will be joining Simon on the judging panel of the US version of his TV talent show The X Factor.

Simon was handling details on the high profile arrangements up to the very last minute.

"Right up until today, I wasn't 100 percent sure it was going to happen or not," Simon told Extra. "Because when you're negotiating with people like that who are big stars, you don't just sign the contract overnight. There's stuff you've got to sort out."

Although Britney was eager to take on her new post from the outset, her representatives weren't too keen on rushing into contract.

"I remember her manager, Larry, saying, Britney wants to speak to you' and about a week had passed and she got on the phone and said Simon, I want to tell you I'm in, and we have a deal.' [Her manager] said, We haven't quite got a deal yet!'... She spoke too soon," he recalled.

Britney is said to have signed a $15 million deal to take up this position. (C) Cover Media