Simon Cowell -- 'The Voice' is Afraid of Britney Spears

Simon Cowell is furious at NBC for trying to sandbag next week's premiere of "X Factor" by adding a Wednesday installment of "The Voice" ... declaring, "This is a cynical, cold-hearted, unprofessional way of doing business."

Simon tells TMZ, he thought he had a "gentleman's agreement" with NBC ... that "The Voice" would run Mondays and Tuesdays, and "X Factor" would run Wednesdays and Thursdays on FOX ... so fans could watch both.

Simon says NBC is turning on its own audience, forcing them to choose between Christina Aguilera's show and Britney Spears' new show. And speaking of them, Simon says, "Britney's not going to appreciate the fact that Christina -- who has been a bit of a rival -- isn't allowing Britney to have a night of her own."

Simon tells TMZ, he believes the move is as personal as it gets, because "The Voice" came to England and went head-to-head with Simon's show, "Britain's Got Talent" and got its ass kicked.

Simon says, "These guys have really got it in for us. It's nothing short of dirty tricks."

Cowell adds, "The reason they've done this is they don't want people to see 'X Factor' because they've heard how good the series is. They don't want their audience to see Britney Spears. They don't want their audience to see Demi Lovato."

Simon says he doesn't think Britney knows yet, but when she finds out "She's going to be devastated because she's put her heart and soul into this."