Ryanair vowed to appeal Wednesday's judgment by the United Kingdom Competition Commission. Ryanair chief Michael O'Leary called it "bizarre."
The commission ordered Ryanair to reduce its 30 percent holding in Aer Lingus to 5 percent to eliminate what it described as Ryanair's ability to "impede or prevent" Aer Lingus' acquisition by any other airline.
Ryanair has launched three failed takeover bids since Ireland's government privatized the state-owned airline in 2006.
Ryanair is Aer Lingus' largest shareholder, while the government has retained a 25 percent stake. The government says it's willing to sell its shares, but not to Ryanair.
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