The French group called the decision by the UK Competition and Markets Authority (CMA) "absurd" on Friday, but will now have to decide whether to sell the business.
The company will have six months to stop calls by two ships operating under the MyFerryLink name.
The ban arises from its acquisition of three ro-paxes from defunct line SeaFrance in 2012.
The original competition decision was made last year, but has been subject to review over jurisdiction questions.
The
