The £1.9bn merger between National Express and Stagecoach is being investigated by the Competition and Markets Authority (CMA).
The regulator has served an initial enforcement order, which prevents the transport firms from fully combining or offloading businesses while it probes the deal.
Stagecoach said the move will delay the planned sale of the marketing, retail and customer service operations of its inter-city coach businesses to ComfortDelGro Corporation.
