An affiliate of Sprint Nextel Corp., iPCS Inc., said Monday it is seeking to block Sprint from morming a wireless broadband company with Clearwire Corp.

Schaumburg, Ill.-based iPCS, with 640,600 subscribers in seven states, said three of its subsidiaries has filed suit in Cook County Circuit Court in Illinois, alleging the service would compete with iPCS within its markets and therefore violates an exclusivity agreement Sprint signed in 1999.