The competition watchdog has ordered Ryanair to cut its stake in rival Irish airline Aer Lingus from 29.8 to 5 per cent.
Ryanair has blasted the final decision by the Competition and Markets Authority (CMA) as "ridiculous" and "manifestly wrong".
The budget carrier said it will now take its case to the Competition Appeal Tribunal and to the UK Supreme Court.
Should a hearing be granted by the latter, it is likely to delay IAG's takeover of Aer Lingus, which was approved by the Irish government last month (see news, May 27).
Ryanair spokesman Robin Kiely said: "Today's CMA decision rejecting Ryanair's request to review its order to divest Ryanair's 29.8 per cent minority stake in Aer Lingus is manifestly wrong and flies in the face of the current IAG offer for Aer Lingus.
"When the only basis for the CMA's original divestment ruling was that Ryanair's minority shareholding was or would prevent other airlines making an offer for Aer Lingus, the recent offers by IAG for Aer Lingus totally disprove and undermine the bogus theories and invented evidence on which the CMA based its untenable divestment ruling."
In 2013, the UK Competition Commission (now the CMA) investigated Ryanair's 29.8 per cent shareholding in Aer Lingus to see if it prevented that carrier from merging to remain competitive (see news, August 2013).
It concluded Ryanair's shareholding could "substantially lessen" competition on routes between Ireland and the UK, and told it to slash its stake to 5 per cent.
Ryanair appealed the findings, but the UK Court of Appeal upheld the decision in February this year (see news, February 13).
The CMA then gave its final decision today — again ordering Ryanair to slash its stake in Aer Lingus.