Concord handled outdoor planning and buying for all Optimedia clients, including British Airways and L'Oreal. However, outdoor arrangements for almost half of the clients will move to Meridian, a 50/50 joint-venture backed by Posterscope and Zenith.
However, Entertainment Film Distributors, the household goods company SC Johnson, Fidelity and Go, now part of easyJet, are expected to stay with Concord.
The move of clients over to Meridian will be conducted on a gradual basis.
There are financial benefits for Zenith Optimedia through increased revenues for Meridian, but sources also claimed that there would be other benefits for clients.
Optimedia's decision is the latest in a series of account moves in the outdoor market. Earlier this week, Mediaedge: cia centralised its £37 million outdoor spend into Portland Outdoor from Outdoor Focus.
Last month, OMD UK moved its outdoor buying into Posterscope from Concord's sister specialist, Outdoor Connection. Walker Media also moved spend from clients including Five into Posterscope from Concord.
Along with Poster Publicity, Concord's parent company, The Alban Group, is one of two remaining independent UK poster specialists facing increasing pressure from poster companies owned by advertising networks.
Nigel Mansell, Concord's managing director, implemented management changes earlier this year at Concord. It created three business divisions to run its accounts. Kate Cochrane was appointed to run the Masterfoods and John Ayling & Associates accounts, Louise Golbourn took responsibility for the Nestle and Walker Media briefs and Nicole Guerretta handled the Unilever, Hyundai and Optimedia business. All three were promoted from board account director to positions as managing partners.
Meridian Outdoor was established eight years ago to provide specialist outdoor planning resources to Zenith, Equinox, Saatchi & Saatchi and Bates UK.
Mansell was unavailable for comment.