
The account includes the media business for the Daily Mirror, Sunday Mirror and The People.
The decision to opt out of the pitch process comes as ZenithOptimedia is already battling in a number of pitches, including Molson Coors, Mars/Wrigley and Reckitt Benckiser.
WPP's Mediaedge:cia and Walker Media are believed to be among the agencies declaring an interest in the MGN business.
The review process is being led by MGN's new marketing director Lorraine Fraser and is expected to be completed by the end of the year.
The Mirror Group spent £6.2m in the 12 months to 31 August, according to The Nielsen Company, with the bulk of the budget being directed towards TV.