Years of simmering ill will in the $600 billion global ad market just broke into open conflict: America’s top advertisers have accused Madison Avenue’s biggest agencies of secretly pocketing rebates from media outlets, with ad firms striking back harshly at what they called a flawed and unproductive attack.

In an eagerly awaited study, a group representing U.S. advertisers accused big ad firms Tuesday of what essentially amounts to receiving kickbacks. Media buyers connected to the world’s biggest ad agencies get rebates when they buy large blocks of ad space—many of which they neither disclose nor pass along to their clients—according to the study, commissioned by the New York-based Association of National Advertisers.

Read the full Crain’s article here.