AT&T, Verizon and several other major advertisers are suspending their marketing campaigns on Google's YouTube site after discovering their brands have been appearing alongside videos promoting terrorism and other unsavory subjects.
The spreading boycott confronts Google with a challenge that threatens to cost it hundreds of millions of dollars.
YouTube's popularity stems from its massive and eclectic library of video, spanning everything from polished TV clips to raw diatribes posted by people bashing homosexuals.
But that diverse selection periodically allows ads to appear next to videos that marketers find distasteful, despite Google's efforts to prevent it from happening.
Earlier this week, Google vowed to step up its efforts to block ads on "hateful, offensive and derogatory" videos. But that promise so far hasn't appeased AT&T, Verizon and other advertisers.