The Competition Commission is to investigate Ryanair's near 30% stake in Aer Lingus.
The UK's Office of Fair Trading has referred the case to the commission.
It will examine whether the acquisition will result in a substantial lessening of competition in airline services between Ireland and the UK.
The OFT started an investigation into Ryanair's stake of 29.8% in Aer Lingus in October 2010 but its probe has been suspended twice.
The first time was because the OFT had insufficient information with which to proceed and the second time was due to a legal challenge by Ryanair, which was dismissed by the Court of Appeal in May.
The OFT has stated that Ryanair and Aer Lingus are, by far, the two largest scheduled airlines operating between the UK and the Republic of Ireland by passenger traffic.
The CC is expected to publish its final report by the end of November 2012.