(Adds details on valuation range)
MILAN, April 30 (Reuters) - Atlantia ATL.MI will ask shareholders on May 31 to assess an improved offer for its motorway unit presented by state investor CDP and its partners, the group controlled by Italy's Benetton family said late on Friday.
Negotiations between Atlantia and a consortium comprising CDP, Macquarie MQG.AX and Blackstone (NYSE:BX) BX.N are part of protracted efforts to end a dispute over the future of the group's Autostrade per l'Italia unit following a deadly bridge disaster.
The sweetened offer submitted on Thursday includes a ticking fee worth up to 230 million euros ($276 million), assuming the deal closes between the end of this year and March 2022, according to a report Atlantia's board has prepared for the shareholders' meeting, published on Saturday.
The ticking fee lifts the bid's valuation of Autostrade to 9.3 billion euros from 9.1 billion euros previously.
"That is not far from the bottom of the range the board has estimated for the whole of Autostrade, considering that the low end of such range stands at 9.3 billion euros to 9.5 billion euros," the report said.
Based on opinions provided by financial advisers and depending on the different calculation methods used, Atlantia's board has valued Autostrade per l'Italia at between 9.3 billion euros and 11.5 billion euros, the report showed.
The future of the unit has been in doubt since a bridge managed by Italy's leading motorway operator collapsed in the city of Genoa in August 2018, killing 43 people.
The consortium's offer is the only firm option currently on the table as a rival bid by Spain's ACS ACS.MC is less advanced and presents "significant risks and uncertainties," Atlantia said in its statement on Friday and in Saturday's report.
The only real alternative to the sale of a stake in Autostrade to press on with pending lawsuits, Atlantia said.
The company's board will decide on the CDP consortium's offer by June 11, following the shareholders' evaluation. ($1 = 0.8321 euros)