The Authority may, upon consideration of a notification, carry out a full investigation and, on completion, determine that the merger or acquisition may be put into effect, may not be put into effect, or may be put into effect subject to specified conditions, on the ground that the result will or will not be to substantially lessen competition in markets for goods or services in the State. "Appropriate date" has the same meaning as in section 19. A conditional determination must include a requirement that the merger or acquisition be put into effect within 12 months. Where a merger put into effect without a prior Commission determination is found on full investigation to substantially lessen competition, the Commission may order unwinding or dissolution or require steps to restore the pre‑merger position where unwinding is not possible.

A written determination must state the reasons for it and include a report. The Authority must furnish the written determination to the notifying undertakings within 120 working days after the appropriate date and publish it, with regard to commercial confidentiality, within 60 working days of making the determination. The 120‑day period is suspended where the Commission makes requirements under section 20(2) within 30 working days of a section 21(2)(b) determination, with detailed rules governing suspension, certification, requests for additional information and deemed compliance; if proposals under section 20(3) are submitted during a full investigation certain time limits are extended to 135 working days. The Authority shall have regard to relevant international obligations of the State.