Italy's competition authority has ruled that Alitalia's tie-up with Air One monopolises the Rome Fiumicino-Milan Linate route, and that the carrier must surrender slots.
The authority states that Alitalia "does not suffer any competitive pressure" from other airlines because of the difficulty in obtaining slots at Linate.
It has not clarified the number of slots required but says Alitalia must put forward a remedial proposal within three months, in order to remove the competitive constraint by 28 October.
While a high-speed rail service operates between the two cities, the authority says this is "still not sufficiently capable" of shifting substantial passenger demand to rail, or influencing the way Alitalia operates.
"To remove the market power of Alitalia on Linate-Fiumicino, it is necessary to have an effective competitive constraint," it adds, stating that this requires the presence of another airline.
"In order to provide a credible alternative to the incumbent, the new carrier would need to have a sufficient number of slots," says the authority.