The advertising firms withholding data, which also include Grey Global, Publicis and Havas's Euro RSCG arm, have taken legal advice recommending they do not provide details of their PR brands for projects, including the PRWeek Top 150 league table, widely seen as a barometer of the UK industry.
The groups -- which own many of the global PR networks -- are acting in accordance with new disclosure rules from the US's Securities & Exchange Commission.
Omnicom diversified agencies group European CEO Anthony Wreford said: "We recognise there's a meeting of interests between us and league table publishers, and that the rankings are a measurement device for agencies, but we face forces beyond our control."
The groups are reacting to changes in disclosure regulations brought about by the the Sarbanes-Oxley Act, a corporate reform law passed in the US last summer. The SEC adopted a regulation in January designed to stop financial disclosures that do not adhere to the accounting standard known as the Generally Accepted Accounting Principles.
PRCA chair Graham Lancaster slammed the SEC ruling: "This seems to be an unwitting consequence of a well-intended act which denies firms access to crucial industry data."
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