The move came into force at the beginning of the year and follows the decision by Manchester-based MediaVest to move its outdoor planning and buying to Posterscope from Kinetic.
Arena BLM, as it was previously known, had billings of £50.4m in 2009, according to The Nielsen Company. With outdoor at around 10% of the market, the account could be worth up to £5m, though this will depend of the agency's client mix.
In November 2010 Arena Media retained the £12m Thomas Cook media planning and buying business, beating Aegis Media's Carat after a lengthy pitch process that began in July.
In May 2010 Havas rebranded Arena BLM to Arena Media to tie it closer to the rest of its network. BLM Media was founded 20-years ago and was one of the last independents to sell when the founders agreed a deal with Havas in 2008.
The move of Arena Media to Posterscope aligns it with sister Havas media agency MPG, which already works with Posterscope. Posterscope also handles the Omnicom media agencies and jointly owns Meridian Outdoor with Publicis Groupe's ZenithOptimedia.
Kinetic works on the outdoor media accounts for WPP agencies MediaCom, Mindshare, MEC and Maxus, and the Publicis Groupe-owned StarcomMediaVest Group.
Spokespeople for Posterscope, Kinetic and Arena Media declined to comment.