The 22-year-old TOXIC singer is currently on the European leg of her Onyx HOTEL roadshow, which has been plagued with negative feedback from a wide range of critics.
And Cowell admits he was far from impressed with Spears's body when he saw the part of her show where she cavorts around in underwear.
He says, "I kind of like this about Britney - when she makes great pop records like BABY ONE MORE TIME and TOXIC, they're great pop records.
"I shouldn't say what I'm about to say...but the shot of her in her underwear onstage, I would've gone to the gym a bit more before I did that. Sorry, but true."
And to make matters worse for Spears, Cowell gives the star's chart rival Christina Aguilera a far more glowing review.
He adds, "Great voice, great attitude. I just think this girl is a star. I really like her.
"Where (Britney's) different from Christina is that she is someone who's so reliant on great pop records produced and written for her, whereas Christina is a bit more kind of old school - just an unbelievable singer."21/05/2004 00:07