Stan Myerson, the joint managing director of Northern & Shell and Express Newspapers, said: "The legal dispute will not impact on the group's relationship with MediaCom in the UK. There are no plans for a review."
Northern & Shell's writ came in response to WPP's decision to sue Desmond for $10 million in the US and Canada for non-payment of media billings for advertising to launch the US version of OK! last summer.
Desmond hit back by lodging his own claim for $5.5 million. He claims that WPP executives reneged on a deal linked to the amount of advertising space they would buy in OK! during its US launch.
According to Desmond, WPP agreed to place 15 pages of ads per week in the new title.
Express Newspapers' £13 million media account was shifted to MediaCom from its WPP sister agency MindShare last year. It was moved to allow MindShare to bid for News International's media account, which it won.
A MediaCom spokesman confirmed the legal actions in the US and Canada, but declined to comment on the agency's relationship with Express Newspapers in the UK.