* Sale to be worth up to 2.65 bln euros, excluding earn outs
* Deal could pave the way to wider network deal with TIM
* Stake sale expected to be completed by June 30, 2021 (Adds Treasury official, Macquarie comments, detail)
By Stephen Jewkes and Elvira Pollina
MILAN/ROME, Dec 17 (Reuters) - Italy's biggest utility Enel ENEI.MI kicked off the sale of up to 50% of fast broadband company Open Fiber to Australian infrastructure fund Macquarie on Thursday, in a move that could open the way to the creation of a full-fiber national network.
Enel, which jointly controls Open Fiber with state lender CDP, said it had given a mandate to its CEO to sell between 40% and 50% of the company to the Australian fund by June.
It said Macquarie would pay 2.65 billion euros ($3.3 billion) for a stake of 50% but added it included the transfer to the fund of a shareholder loan worth 270 million euros.
The deal also includes a series of earn-out clauses, including one linked to the positive conclusion of a legal dispute between Open Fiber and phone incumbent Telecom Italia (TIM) TLIT.MI .
A sale by Enel of its stake could clear the way for a government-sponsored plan to merge Open Fiber with the last-mile grid unit of TIM and create a unified ultrafast broadband network.
Enel said another earn-out clause in the agreement revolved around implementation of this network.
CDP, which is the second-biggest shareholder in TIM behind French media giant Vivendi VIV.PA , would emerge as a prominent investor in the combined entity.
"We judge it positively. It went as we hoped - a decision on the sale by Dec 17 and control to CDP to speed up the unified network plan," a Treasury official said.
Sources have previously said Rome wants Macquarie to buy less than 50% so CDP, which has a right of first refusal on the Enel stake in case of a sale, can take control of Open Fiber and make sure the blueprint for a single network is respected.
Enel said any deal to sell its whole 50% stake would be subject to Macquarie and CDP sharing "modification of certain aspects that currently regulate the governance of Open Fiber."
The utility also said the sale was conditional on Open Fiber allowing Macquarie to share due diligence data with a small number of co-investors so it could syndicate the asking price.
"(We) welcome the opportunity to enter the next phase of discussions with Open Fiber's shareholders and stakeholders," Macquarie Infrastructure and Real Assets Senior Managing Director Jiri Zrust said.
($1 = 0.8156 euros)