The report is set to be made public early next week by the Association of National Advertisers, which includes companies such as Apple Inc., Wal-Mart Stores Inc. and General Motors Co., according to a person close to the investigation.

It will lay out a variety of ways in which global ad agency networks used the combined buying power of advertisers to pad their bottom lines, according to the person, who asked not to be named before its release.

The findings spotlight an issue that has long been hidden in the background of client-agency relationships, opening up a debate over whether advertising conglomerates are agents working on behalf of advertisers or counter-parties in today’s complex media marketplace. Ad agencies have come under pressure from a shift to digital marketing, which threatens to displace them as a middleman between advertisers and media outlets.

Read the full Bloomberg article here.