CBS Sues ABC -- You'll PAY for Ripping Off 'Big Brother' [Update]

CBS wasn't bluffing ... 'cause the network has made good on its threat to sue ABC over an upcoming reality show which CBS claims is a blatant rip off of "Big Brother" ... and it's NASTY.

As we previously reported, CBS fired off a cease-and-desist letter to ABC last week ... claiming "Glass House" is "strikingly" similar to "Big Brother" -- a bunch of people living in a house and competing for prizes.

Now, in a lawsuit filed in federal court in L.A., CBS states, "If imitation is the sincerest form of flattery, then CBS should take pride in ABC's latest reality television project, 'Life in a Glass House.'"

In the suit, CBS refers to "Glass House" as a "carbon copy" of "Big Brother" -- and calls ABC's decision to green light the show an "obvious attempt ... to capitalize on its unique success."

CBS claims its trying to stop ABC from committing a blatant theft of its copyrightable expression, trade secrets and other confidential information.

CBS also claims ABC has hired at least 19 former "BB" staffers to work on "Glass House" -- and believes the employees are actively disclosing CBS's trade secrets and confidential information to produce the show. CBS also believes at least one "Glass House" exec has been trying to poach current "BB" employees.

CBS wants at least 3 former high level "Big Brother" employees who signed with "Glass House" to fork over $500,000 EACH for allegedly violating a confidentiality agreement they had with CBS.

CBS is also suing ABC for unspecified damages.