The procedure was launched after Ryanair ignored an order to update its website and standard emails to ensure Italian customers were made fully aware of their rights to re-booking, re-routing and compensation in the case of flights being cancelled, the antitrust regulator said in a statement.
The company challenged the order, which required the changes to be made within ten days, but its appeal was rejected by a tribunal last month.
Several press reports have claimed Ryanair cancelled flights as a result of staff shortages triggered by pilots and cabin crew leaving the company, or being required to take holidays before the year end.
The company has denied these reports, saying it was the result of a "pilot rostering failure".
More than 700,000 people have seen their travel plans ruined and the airline has been slammed by consumer groups and regulators for its handling of the crisis, particularly in relation to the rights of customers whose flights are cancelled.
Britain's Civil Aviation Authority reprimanded Ryanair's CEO Michael O'Leary in September for wrongly claiming that the airline was not obliged to re-route cancelled passengers on other airlines.
The company, Europe's biggest airline by passenger numbers, said on Monday that it carried 9.3 million travellers last month, up 6 percent on November 2016.
That was its slowest rate of growth in three years but the company is sticking by its pre-crisis profit forecast despite the cancellations.