The investigation, reported by Italy's Corriere della Sera, concerns Ryanair's hiring of 220 staff at its Bergamo hub near Milan while putting them on the books in Dublin, where tax rates are far lower.
Estimating the loss to the Italian state at €12m in taxes, the magistrate Maria Mocciaro gave the airline 90 days to settle its account, an offer Ryanair turned down, the newspaper reported.
Although the airline's Bergamo staff are required to live near the airport, Ryanair claims they work on board Ryanair aircraft and their taxes should be paid in Ireland, the home of the carrier.
A Ryanair spokesman declined to comment on the report, stating the airline would "continue to observe EU tax laws".
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