Shop's rolling contract with the car manufacturer finishes at the end of the year. At that point, sources close to the business say the account will move into the network.
"It's inevitable. The ink is pretty much dry on it," a senior BBDO source said.
The loss will be a serious one for the agency. The account is the largest billing at Shop and spent £13.2 million in the year ending 31 January on above-the-line advertising, according to Nielsen Media Research. The figure represents 76 per cent of Shop's £17.4 million total billings.
Mercedes hired BBDO to refine the creative work of its German lead agency, Jung von Matt, across more than 150 territories in December.