Marriott, the hotel operator, is reviewing its global media planning and buying account, putting MEC on alert.
The review, confirmed by the company to Ad Week, follows the merger last year between Marriott and Starwood Hotels and Resorts.
MEC, which is in the process of merging with fellow Group M agency Maxus, was appointed as lead global media agency for Marriott in 2011.
The account had covered both media planning and buying for all of Marriott's brands, including Marriott Hotels and Resorts, the Ritz-Carlton and Marriott Rewards.