ACS values the whole of Atlantia's Autostrade per l'Italia unit at 9 billion to 10 billion euros ($10.7 billion-$12 billion) and does not mention in its proposal a request for guarantees against legal risks, according to TCI, which was aware of the contents of the letter.
ACS declined to comment.
"We welcome ACS's offer, which is far superior to the offer made by (Italian state lender) CDP and its partners," Jonathan Amouyal, partner of TCI Advisory, told Reuters.
According to Amouyal, ACS said in its proposal that it was open to teaming up with an Italian partner over a deal on the toll road operator.
The new proposal for Autostrade, which was signed by ACS's and Real Madrid soccer club Head Chairman Florentino Perez, cheered up investors in Atlantia.
At 1510 GMT shares in the infrastructure group were up 3% while Milan's blue-chip index was in negative territory.
Last week, CDP and its partners, Macquarie and Blackstone, filed a new proposal to buy Atlantia's 88% stake in Autostrade in a fresh effort to end a dispute triggered by the collapse of a bridge run by the unit.
The consortium's binding offer is based on a valuation of 9.1 billion euros for the whole of Autostrade and asks Atlantia to cover 870 million euros in potential legal risks, sources said.
"The Italian government should allow Autostrade to be sold to the highest bidder without any political interference," TCI said, asking Atlantia's board to explore the ACS offer.
The CDP-led consortium's move on Autostrade has the backing of the Italian government, which has still to give its final blessing to a settlement agreement with Atlantia over the bridge disaster, which killed 43 people on Aug. 14, 2018.
The news of the letter, first reported by the Financial Times, emerged as the board of Atlantia was meeting to review the offer presented by the CDP-led consortium.
Back in 2017, ACS and Atlantia jointly acquired Spanish toll-road operator Abertis in a deal worth more than 16 billion euros.
($1 = 0.8401 euros)
(Additional reporting by Clara-Laeila Laudette in Madrid; editing by Agnieszka Flak and Steve Orlofsky)
By Francesca Landini