The decision, filed after a Friday hearing, said the tender breached antitrust rules, the judicial source said.
Mediapro now has 15 days to appeal the decision. Mediapro was not immediately available for comment.
The tender had already been suspended in mid-April after Sky Italia had requested the court to verify whether the sale respected Italian regulation.
In February, Italy's Serie A soccer league had accepted Mediapro's offer for the rights to broadcast the almost 400 matches of the 2018-2021 seasons at just above 1.05 billion euros ($1.24 billion).
Sky Italia welcomed the decision saying it "made clarity to the benefit of all participants... setting out the requirements for the procedure."
It added in a statement that it was ready to make an offer for the TV soccer rights.
(Reporting by Manuela D'Alessandro in Milan, writing by Giulia Segreti, editing by Steve Scherer and Louise Heavens)
