Italy's competition authority Antitrust has opened an inquiry into Ryanair's decision to charge passengers for hand luggage, which the body described as an "essential" item for passengers.
From November, non-priority customers of the low-cost airline will only be allowed to take one "small personal bag", such as a handbag or laptop case, into the plane's cabin as long as it can fit under the seat in front.
Ryanair announced last month that customers who want to bring more than a small bag into the cabin such as a 10kg holdall or suitcase will have to pay extra charges ranging up to €8.
Hand luggage is "an essential element of transport" so Ryanair, and all other carriers, should include the price in the cost of a plane ticket, Antitrust said in a statement cited by the Italian press.
It added that the new Ryanair policy could amount to unfair commercial practice in that it distorts the final price of the ticket and does not allow a true comparison with other airlines' prices.
Italian consumer associations had complained to Antitrust about the Ryanair decision.
"If its unfair commercial practice on hand luggage is confirmed, Ryanair... should reimburse all its customers who suffer unfair additional costs", the association Codacons said in a statement, promising to take the matter to court if necessary.
The Italian inquiry heaps more negative publicity on Ryanair whose shareholders delivered a blow to the airline's chairman amid widespread strike action by European staff that has rattled confidence in the company.
Chairman David Bonderman was re-elected but only with 70.5% of the vote at yesterday's Annual General Meeting - a drop from last year's assembly where he garnered a 89.1% endorsement.
"Questions about the company's business model and governance now pose a threat to shareholder value," said the chairman of one of the shareholders, the Local Authority Pension Fund Forum.
Ryanair has said that no customer has a right to "unlimited carry-on bags".
In a statement it said: "For safety reasons, most short haul aircraft cannot accommodate two carry-on bags for each customer."
The airline also said that it looks forward to cooperating with Antitrust in the inquiry.