Britain’s competition watchdog says Ryanair must sell its shares in Irish rival Aer Lingus, but Europe’s dominant budget airline has dismissed the ruling and vows to appeal.

The U.K. Competition and Markets Authority upheld its own April ruling that Ryanair’s nearly 30 percent stake in its main Dublin-based competitor restricted Aer Lingus’ ability to take its own decisions. Ryanair became Aer Lingus’ largest shareholder as part of three hostile takeover bids following the 2006 privatization of the former state airline.

In Thursday’s judgment the lead investigator, Simon Polito, says the recent friendly takeover bid for Aer Lingus by British Airways parent IAG demonstrates Ryanair’s inappropriate influence over Aer Lingus — because IAG won’t complete its offer unless Ryanair accepts it.

Ryanair says it will appeal to the U.K. Supreme Court.

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Photo Credit: Aer Lingus check-in counters at Dublin Airport Swire / Flickr