The Competition Authority is to carry out an investigation of the proposed takeover of NTL by rival cable operator, Liberty Global.
As Liberty Global already owns rival cable company, Chorus, the authority must determine whether the merger will substantially lessen competition in the market for cable pay-TV, telephone and Internet-related services.
The competition watchdog's preliminary investigation was unable to conclude whether the propose merger would have anti-competitive effects.
The Competition Authority has until 8th December, 2005 to make a final determination. However, Minister for Trade and Enterprise Micheál Martin will have the final say as media mergers enjoy special status in competition law.
Mr Martin has a further 30 days from the authority's published decision to give the green light or add any conditions to the deal.