The Time Warner Cable chief confirmed the long-standing rumor that, after 12 years running the No. 2 cable operator, he’ll split at year-end when his contract expires. Directors formally designated COO Rob Marcus as his heir, and elected him to join the board immediately. The transition is taking place at a time when TWC’s own fate is filled with questions: Liberty Media’s John Malone is probing for opportunities to help Charter Communications take a run at TWC. The company also has been weighing opportunities to buy systems, possibly Cablevision or Cox. Meanwhile, TWC is embroiled in a bitter dispute over carriage prices with CBS, which could result in its stations going dark on the cable company’s systems in New York, Los Angeles, and Dallas. Britt, 64, has led the cable industry’s charge against rising programming costs, dropping small channels including Ovation that he said did not carry their weight. But the exec — who rose through the ranks as a numbers guy at Time and then Time Warner — didn’t flinch from spending to upgrade TWC’s lines so they could accommodate two-way communications: That paved the way for the current boom in broadband and interactive TV, contributing to the 270% rise in TWC’s shares over the last four years. TWC lead director N.J. Nicholas praised Britt’s “vision, financial prowess, operating acumen and unrelenting drive.” And Britt, who will stay on the board in a non-executive role, lauded Marcus as “an accomplished executive” and “strong strategic decision maker.” Marcus, 48, is a lawyer and best known for his M&A and deal making expertise. He helped to lead the 2009 effort by Time Warner to spin off the cable systems.