"We will investigate whether regional and local authorities in Germany, against the rules, gave an unfair advantage to Ryanair over its competitors," EU Competition Commissioner Margrethe Vestager said in a statement.

Under the deal, Ryanair received between 2009 and 2017 training aid and funding for a crew and pilot school and for a maintenance hall at the airport, the Commission said.

Ryanair said it was confident that the Frankfurt-Hahn agreement fully complied with EU state aid rules.

"The EU has previously investigated this matter in 2014 and ruled that there was no state aid involved in the Ryanair-Frankfurt-Hahn agreement," the airline said in a statement.

The previous investigation related to arrangements before 2009.

The EU executive opened a similar investigation in July over a deal between Ryanair and the French airport of Montpellier.

The Commission said it will also investigate possible state aid to airport operator Flughafengesellschaft Frankfurt-Hahn GmbH (FFHG), which between 2009 and 2017 was controlled by the German state of Rhineland-Palatinate.

(Reporting by Francesco Guarascio; Editing by Philip Blenkinsop and Susan Fenton)