The move follows news that the brand is in talks with the specialist retail ad agency Yellow Door about a new campaign.
The Beattie McGuinness Bungay co-founder is credited with turning round the fortunes of French Connection after devising the controversy-courting fcuk brand campaign for the label in 1996.
Beattie said: "It's not every day you resign an account you've worked on for ten years. But fashion being fashion, standing still is not an option. I feel the time is right for us to move on."
French Connection was a founding client of BMB when the agency launched in May 2005.
A spokeswoman for French Connection would not comment.