The Publicis Groupe chairman, Maurice Levy, offered Farrell the role of running Specialised and Marketing Services Agencies, when D'Arcy's closure was announced in October.
Farrell delayed giving a decision but will now press ahead with a potential shake-up of Publicis' below-the-line operations.
The future shape of its marketing services operations has yet to be decided following the merger. One possibility that is expected to be discussed is a merger of Arc, D'Arcy's recently created below-the-line network, and iLeo, Leo Burnett's direct marketing and digital operation. Leonardo is Burnett's UK below-the-line operation.
Farrell was instrumental in the creation of Arc, which
was formed earlier this year.
Before running D'Arcy, he was responsible for IMP, which formed part of the operations rebranded as Arc.
Publicis' other UK below-the-line interests include Publicis Dialog, which provides direct, design and digital marketing, and Triangle, the sales promotion and direct marketing agency affiliated to Saatchi & Saatchi.
At the time of the merger, Levy said of Farrell's potential new role: "It's not a snub to John. He has a unique experience in marketing services, an area in which we want to develop much more aggressively. The job is real."
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