* ACS confirms bid for Atlantia 's stake in Autostrade
* Offer values Autostrade at 9 billion-10 billion euros
* Follows last week's binding offer from CDS-led consortium
* Autostrade didn't examine ACS offer at Thursday meeting -source (Recasts, adds sources)
MADRID, April 9 (Reuters) - Spanish infrastructure group ACS ACS.MC confirmed on Friday it had made an offer for Atlantia's ATL.MI 88% stake in Italian motorway unit Autostrade per l'Italia, a move that risks unsettling a Rome-backed plan to reclaim the asset.
ACS said in a statement it had sent a letter to Atlantia's board on Thursday declaring its interest in a business that it values at 9 billion-10 billion euros ($12 billion).
Atlantia, controlled by the Benetton family, has been under pressure to sell its stake in Autostrade since the 2018 collapse of a motorway bridge it runs in the town of Genoa that caused 43 deaths.
Autostrade, which runs half of Italy's motorway network, was privatised in 1999 when the Benettons took the helm.
Italian state lender Cassa Depositi e Prestiti (CDP) is already heading a team of investors, including Macquarie MQG.AX and Blackstone (NYSE:BX) BX.N , which presented a binding bid for the unit last week.
A source close to the consortium dismissed the ACS bid as vague and non-committal.
"Atlantia has an offer on the table that can be signed in days, the other is woolly and a long way off," the person said. "For sure, it must be seen as an alternative offer."
ACS, which has not said how it would structure any deal, said in its statement earlier on Friday it was open to team up with other investors including CDP.
Another source close to the matter said the Atlantia board had not examined the ACS offer at a meeting on Thursday because the letter was short on detail but added the board had asked its advisers to explore ACS's intentions.
"There's no timeframe for these explorative talks," the source said.
POTENTIAL LEGAL RISKS
ACS's statement fuelled investors' appetite for the Italian infrastructure group with shares rising more than 1% on Friday, adding to Thursday's gains. firms UBS, Alantra and Mirabaud welcomed the prospect of the acquisition, though some noted likely political opposition from Italy, as well as possible strain on ACS.
"It would have to raise around 5 billion euros in debt... (putting) too much pressure on ACS's balance sheet," Mirabaud analysts said.
CDP and its partners' bid is based on a valuation of 9.1 billion euros for the whole of Autostrade. It has asked Atlantia to cover 870 million euros in potential legal risks linked to the 2018 bridge disaster, sources had said.
"The preliminary valuation of Autostrade by ACS is not significantly higher than that of the CDP-led consortium... and it is not excluded that following due diligence ACS may also ask for guarantees for legal risks," Italian broker Bestinver said.
ACS has been Atlantia's partner in Spanish toll road operator Abertis since the two groups' joint acquisition of the company in 2017.
Florentino Perez, chairman of ACS and Real Madrid soccer club, said on Tuesday he hoped to create a pan-European group of road operators with his Italian partner. ($1 = 0.8415 euros)