* NBC schedules third night of "The Voice" on Wednesday
* Britney Spears makes "X Factor" judge debut same night
* Ratings war heats up in crowded TV talent show field
By Jill Serjeant
LOS ANGELES, Sept 6 (Reuters) - An angry Simon Cowell hit back at NBC on Thursday for a late decision to pit singing contest "The Voice" in direct competition with his revamped "The X Factor" season premiere next week, calling it a mean-spirited spoiling tactic.
NBC fired a new round in the ratings war in the crowded TV talent show market by announcing it would air a third night of "The Voice" next Wednesday - the very night "X Factor" returns to Fox with new judges Britney Spears and Demi Lovato.
"It is a spoiling tactic. They (NBC) don't want people to see this first episode. I think it is mean spirited and I hope and I pray it backfires on them because this is one of the best shows we have ever made," Cowell told reporters on a telephone conference call.
"I am pissed off about it, because I think there's a kind of gentleman's agreement (not to go head to head). But we have to rely that the viewers will make the right selection," the sharp-tongued British TV entrepreneur said.
"The Voice," with celebrity musician judges Christina Aguilera, Cee Lo Green, Blake Shelton and Adam Levine, has been one of the shining lights in struggling NBC's schedule since it made its debut in the spring of 2011.
The network will now air the contest twice a year, starting with a two night premiere scheduled for Sept. 10 and 11. With just four days to go before the start of the new TV season, NBC said "The Voice" will also extend to Wednesday next week.
After a disappointing first season last year that ended with creator Cowell firing two judges and the show's host, "The X Factor" returns on Sept. 12 and 13th.
Reality singing show fans will now have to choose which show to watch on Sept. 12. In another curious juxtaposition, Cowell's "America's Got Talent" hit summer show on NBC will broadcast its two-night finale next week directly opposite "X Factor."
"I think there is a lot of interest in our show because of Britney and Demi, so we will see what happens, but I have learned, don't make any predictions," Cowell said.
The two shows already pit one-time Disney child stars Aguilera and Spears against each other.
"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," Cowell told celebrity website TMZ. "She's going to be devastated because she has put her heart and soul into this."
Comcast Corp-owned NBC has been at the bottom in overall viewership for several years among the four leading U.S. networks. But it rose to third in the 18-49 age group most prized by advertisers in the 2011-12 TV season, largely on the back of "The Voice," "Sunday Night Football" and its broadcast of the 2012 Super Bowl.
Fox, a unit of News Corp, has dominated the 18-49 category for eight years, mostly due to annual singing contest "American Idol," but the network has said it may relinquish that position this season as the popularity of "Idol" wanes.
Cowell said the first audition episode of "The X Factor" - which offers a $5 million prize to the winning singer or group - "looks different to anything we have ever made before and I think this is the reason why NBC decided to put 'The Voice' up against us."