The European Commission said it has opened an in-depth investigation into a marketing deal between Ryanair and Montpellier airport in France, to see whether it constituted illegal state aid. 

"We will investigate whether regional and local authorities in France granted an undue economic advantage to Ryanair over its competitors," EU Competition Commissioner Margrethe Vestager said. 

Under the deal, Ryanair received substantial payments in exchange for promoting the Montpellier area as a tourist destination on its website, the Commission said.

In a statement, Ryanair said all of its airport arrangements comply fully with EU state aid rules.

"We are confident that the EU will find our Montpellier deal is in line with EU State aid rules," the statement added.