Britain's Competition Appeal Tribunal has dismissed Ryanair's appeal against a ruling that orders it to sell most of its holding in Aer Lingus.
The airline had challenged a decision by the country's Competition Commission, which ordered it to reduce its stake in its rival to no more than a 5% holding.
In its judgment in August of last year, the authority said Ryanair's 29.82% stake in Aer Lingus gave it "material influence" over the firm and resulted in a substantial lessening of competition.
Following today's announcement, the airline said it noted the ruling and has instructed its lawyers to take the case to Britain's Court of Appeal.
Ryanair had previously said the competition authority's decision was contrary to European law.
The airline's third bid for Aer Lingus was rejected by the European Commission on 27 February 2013.
Its appeal against that decision is now pending before the General Court.