* Atlantia convenes shareholders meeting on spin off on Jan. 15

* Govt confident in an Atlantia-CDP deal by mid-Jan -source

* CDP preparing letter with road map for Autostrade deal-source

ROME/MILAN, Dec 14 (Reuters) - Infrastructure group Atlantia said on Monday state lender CDP has until July to present an offer to buy its motorway assets, but a government source expressed confidence that the two parties will be able to make a deal by mid-January.

CDP, together with co-investors Macquarie and Blackstone, has been in talks since October with Atlantia to buy its 88% stake in toll-road unit Autostrade per l'Italia, but no binding bid has yet been presented.

A deal between the two parties would put an end to a dispute sparked by the 2018 collapse of a motorway bridge operated by Autostrade that killed 43 people.

A deadline for prospective buyers to present a binding bid came and went as Atlantia, which is controlled by the Benetton family, pressed ahead with an alternative plan to spin off and sell or list the stake, while leaving the door open for suitors.

The infrastructure group said in a statement on Monday it had called a shareholder meeting for Jan. 15 to vote on the issue. If CDP and co-investors present an offer by then, the board will examine it and ask shareholders to vote, Atlantia said.

CDP is preparing a letter for Atlantia detailing a road map to come up with a binding offer by mid-January, with an update on proceedings on Dec. 22, a second source said.

If no bid arrives, the group will carry on with a plan that would have Atlantia end up with nearly 63% of a new company that would hold the Autostrade stake. Atlantia would then offer that stake on the market to fully exit the unit.

Atlantia said CDP or any other suitor had until the end of July, when that spin-off is due to be finalized, to present a bid, in which case another shareholder meeting would need to be called to revoke the spin-off.

The CDP-led consortium valued Autostrade at 8.5-9.5 billion euros in a preliminary offer, but potential legal claims and fallout from a probe has since made it more cautious. (Reporting by Giuseppe Fonte, Francesca Landini and Stephen Jewkes; Editing by Sonya Hepinstall)