Earlier this month the Italian government reached a preliminary compromise that left Autostrade with its motorway concession but forced Atlantia to hand over control to state lender CDP and allied investors.

The deal opened the way to settling a bitter dispute set off by the collapse of a bridge run by Atlantia's tollway unit in the northern city of Genoa in 2018, in which 43 people were killed.

But TCI, an investor in infrastructure group Atlantia, said in a letter of complaint that Italy's order that Atlantia sell down its stake in Autostrade "seriously threatened the fundamental principles of the EU Capital Market Union".

TCI demanded that the sale of the unit should be carried out "transparently, in accordance with market standards" and asked the government to launch a competitive auction to sell Autostrade or, alternatively, to demerge it and list it to create the conditions for Atlantia to sell its controlling stake in the motorway unit to CDP at a "fair" price.

The preliminary deal reached between Atlantia and the government was due to be ratified by Aug. 3, just before the inauguration of the new bridge in Genoa, the source said.

Atlantia has picked Bank of America Merrill Lynch, JP Morgan and Mediobanca to advise on the sale of a stake in its motorway unit, two sources close to the matter said, as suitors line up to invest in its Autostrade per l'Italia unit.

By Giuseppe Fonte