Marketers are being urged to re-evaluate the role of media auditors amid concerns over a potential conflict of interest, with WPP CEO Martin Sorrell advising them to use one of the “Big Four” global accountancy firms to vet how agencies spend their clients’ money.

Sorrell singled out Ebiquity, arguing that it is “heavily conflicted” because it has one subsidiary, FirmDecisions, that audits whether agencies have complied with their contracts with advertisers while other parts of the business manage pitches for clients, advise them how to “secure better prices” and audit media spend.

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