Ryanair has lodged a legal case against the European Commission, accusing it of not preventing alleged illegal French state aid for Air France.
Dublin-based Ryanair has long accused the French government of illegally giving Air France 1bn euros ($1.5bn; £696m) over the past seven years.
Ryanair, which first threatened legal action in July, has gone to the European Court of First Instance.
The European Commission was not immediately available for comment.
It has previously stated that it has dealt with Ryanair's complaints both fairly and in accordance with its duties, and that it believes a court would uphold that view.
Other legal moves
"We are calling on the Commission to start promoting competition and stop protecting flag carrier airlines who continue to receive unlawful state aid," said Ryanair's chief executive Michael O'Leary.
Ryanair is currently involved in a number of separate legal challenges.
It is challenging a Commission decision to block its bid for fellow Irish airline Aer Lingus, and is fighting planned flight cuts at Rome's Ciampino airport.
Ryanair is also challenging the French government's demand that the airline applies French working practices for its staff working in France.
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