Ryanair is facing an €8.3 million penalty after losing an appeal against a decision it broke French labour laws.
The court found that Ryanair had broken French laws when it employed around 120 members of crew on Irish contracts to avoid paying making French social security payments.
The case refers to Ryanair staff employed at Marseille airport between 2007 and 2010.
The original verdict was in October last year. Ryanair were fined €200,000 plus damages of nearly €8 million in backdated social insurance and pension payments. The decision was immediately contested.
The airline said that the employees had already paid all their taxes and pension contributions in Ireland over the three-year period in question.
The French appeal court upheld the original decision, but did reduce the amount of damages by €700,000.
Ryanair’s head of communications Robin Kiely told Sky news: "Ryanair will appeal this ruling to the French Supreme Court on the basis that European employment law clearly allows mobile workers on Irish registered aircraft to pay their taxes and social taxes in Ireland.
"We will also be seeking a referral to the European Court of Justice to prevent these attempts by the French authorities to claim social taxes that have already been paid in full to Ireland."