CBS Threatens ABC -- Shut Down 'Big Brother' Rip-Off!

CBS is clashing with ABC over a new reality show -- claiming ABC's newest reality project "Glass House" is a "Big Brother" rip-off ... and if ABC doesn't pull the plug on it stat, CBS will sue.

Lawyers for CBS fired off a cease and desist letter to ABC on Friday, claiming "Glass House" -- about contestants living in a house, filmed round-the-clock, and voted out by viewers -- is strikingly similar to "Big Brother."

CBS says it's no coincidence either -- claiming 18 former "Big Brother" staffers are now working on the new ABC project.

CBS writes, "In the strongest possible terms, we must admonish ABC and anyone involved in the development or production of 'Glass House' that they will be acting at their own peril if they continue to proceed in this manner."