MILAN, April 30 (Reuters) - Atlantia has called a shareholder meeting 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 on Friday. Negotiations between Atlantia and a consortium comprising CDP, Macquarie and Blackstone 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 in August 2018. The sweetened offer the consortium submitted on Thursday includes a ticking fee, lifting the valuation of Autostrade per l’Italia to 9.3 billion euros from 9.1 billion previously, Atlantia said in a statement. Atlantia said the consortium’s offer was at present the only concrete option on the table because a rival bid by Spain’s ACS presented “significant risks and uncertainties.” Atlantia said the only real alternative to the sale of a stake in Autostrade to press on with pending lawsuits. The board will decide on the CDP consortium’s offer by June 11, following the shareholders’ evaluation, Atlantia said. (Reporting by Francesca Landini; editing by Valentina Za)