The Competition Authority has commenced legal proceedings against the Irish Medical Organisation.
A High Court hearing is expected within the next week.
It follows the refusal of the IMO to rescind a decision of its GP Committee last week.
The committee sought to withdraw certain GP patient services in protest at proposed Government cuts to fees paid to GPs under the medical card contract.
The authority said it had filed papers in the High Court seeking a declaration that the IMO's action is prohibited under Irish and European law.
It is also seeking an injunction to require the doctors' union to retract and rescind its decision.