The agencies pitched under the name The Omnicom Consortium against WPP Group, M&C Saatchi and Grey Worldwide.
Royal Mail said it chose the Omnicom companies because they best answered its objective of providing an integrated approach to its marketing.
A Royal Mail spokesman said: "Royal Mail needs to reduce costs of managing agencies. We were looking to get more effective marketing for less money. [The Omnicom Consortium] not only provided an integrated pitch, but one that works, from good quality companies."
The agencies will work together to communicate Royal Mail's new image, which is being developed as part of an ongoing overhaul of the business. The overhaul, including the change of name from Consignia back to Royal Mail, is aimed at stemming its 拢1m-a-day losses.
The campaign, set to break by the end of the summer, will aim to present a unified face to consumer and business customers following the shake-up.
Proximity London chief executive Simon Hall said: "The key task for us will be to ensure a consistent Royal Mail voice is heard by its customers across all communications and media."
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