Announced today, Tanaka Koki has left Johnny's & Associates as of September 30.
According to the official announcement of the office site, due to multiple violations of the agency's rules, they have terminated Tanaka's contract. Tanaka apologised for his transgressions and also commented that he would be continuing his professional activities and make further announcements. KAT-TUN will continue as 4 members, Kamenashi Kazuya, Taguchi Junnosuke, Ueda Tatsuya and Nakamaru Yuichi.
Tanaka Koki's profile has already been removed at the official profile.
You can read the agency's announcement message, Koki and KAT-TUN's messages and apologies at J&A's site in English.
edit: Some people are saying Tanaka resigned before his contract ended, but I haven't seen a source for this.
sources 1, 2, 3
.........i can't say i didn't expect such an outcome but i'm still shocked... ;_;
at the very least... we know they're not disbanding.