The Competition Authority is to launch a Phase Two probe of the proposed acquisition by ferry operator Stena of two routes on the Irish Sea.
The Authority was unable to establish after the first examination if the deal, which sees Stena acquiring P&O's Liverpool/Dublin and Larne/Fleetwood routes, would result in a substantial loss of competition.
The UK Competition Commission said in a provisional decision in November that the transfer of the Liverpool/Dublin route may cause a substantial decrease in competition.