The investigation follows a complaint by consumer group Codacons, which alleged "a specific collusive will" of airlines to raise prices for domestic flights to and from Sicily during the Christmas holidays, the regulator said in its weekly bulletin.
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In a statement to Reuters, easyJet and Wizz Air denied any wrongdoing.
"EasyJet strongly refutes these claims, adheres to relevant laws and regulations and will fully cooperate with the authorities to demonstrate it has always acted lawfully and competitively in the best interests of consumers," it said.
Wizz Air said it "strenuously denies the allegations of any misconduct", adding that it had "always acted in line with the relevant applicable legislation" and was ready to work with the authorities and support the ongoing inquiry.
ITA Airways also denied wrongdoing. "(There is) no cartel whatsoever," chief executive Fabio Lazzerini was quoted as saying by Italian news agency AGI, adding that the company is willing to cooperate with all of the authorities.
The cost of flights is particularly sensitive in Sicily, many natives of which live and work in northern or central Italy and typically wish to travel back to the island to spend Christmas with family and friends.
The antitrust authority said its inquiry would be completed by the end of 2023.