Ryanair will go to court in London next week to persuade judges that it should be allowed to hold on to its near 30pc stake in Aer Lingus.
The UK Court of Appeal will be hearing the case and it will be another key milestone in Ryanair's seven-year battle with regulators in Britain and the European Commission over its efforts to buy its smaller rival.
Lawyers for Aer Lingus will also make a case in court in support of the UK Competition and Markets Commission (CMC) ruling last year that Ryanair must sell virtually all its shares in the former state-owned airline.
That could leave Ryanair owning just 5pc of Aer Lingus, but it has already conceded that it no longer has any real interest in buying Aer Lingus.
The hearing next week is pencilled in for just five hours, but it's likely to take almost two days as the three sides make their arguments to three judges.
While there's an outside chance that a ruling could be made before Christmas, it's more likely to be sometime in the New Year before the court makes its decision known.
It could conceivably take five or six months for a ruling to be delivered.
The CMC ruled last year that Ryanair, which is headed by chief executive Michael O'Leary, must sell all but 5pc of its stake in Aer Lingus due to competition concerns.
Ryanair appealed the decision that would force it to sell to the Competition Appeals Tribunal, which earlier this year upheld the Commission's decision.
Ryanair then took its case to the Court of Appeal.
If it fails to sway Appeal judges next week, Ryanair's last possible avenue of recourse in the UK will be to the Supreme Court.
Mr O'Leary has previously described the UK probe into its Aer Lingus stake as a "charade".
Ryanair has argued that as it is still awaiting an appeal to a European Commission ruling that blocked its last Aer Lingus takeover attempt, that UK regulators shouldn't be able to force it to sell its Aer Lingus shares.