Britain's Court of Appeal has rejected an attempt by Ryanair to block regulators investigating its shareholding in rival Aer Lingus.
Ryanair, Europe's largest budget carrier, argued that it was not within the UK Competition Commission's jurisdiction to investigate its near 30% stake in Aer Lingus.
The failed appeal runs alongside a new takeover bid from Ryanair for Aer Lingus - made in June - as it continues its pursuit of Aer Lingus.
Britain's Office of Fair Trading had ruled that Ryanair's ownership of a minority stake in Aer Lingus threatened competition in the UK. It asked the Competition Commission to consider sanctions.
The Competition Commission - unlike the OFT - has the power to force Ryanair to divest its stake.
Ryanair mounted a public takeover for all of Aer Lingus in October 2006, but the European Commission investigated the bid and decided to prohibit it in June 2007.
The commission ruled, however, that Ryanair could not be forced to sell its stake, since Ryanair did not have control of Aer Lingus.