With its 12.9% stake Mediobanca is the single largest shareholder in Generali, followed by Italian billionaires Francesco Gaetano Caltagirone and Leonardo Del Vecchio.
The two tycoons have struck a pact to consult over decisions concerning Generali and people with knowledge of the matter have said they will back a different CEO candidate if Generali's board proposes appointing current boss Philippe Donnet for another mandate.
Generali's board meets on Monday to discuss whether to submit its own slate of nominees, including Donnet, ahead of a general meeting next spring to name new directors.
Caltagirone, a construction magnate, and Del Vecchio, founder of Ray-Ban owner Luxottica, have come to control a combined 12.5% stake in Generali after another investor, Fondazione CRT, last week agreed to join their pact.
Mediobanca said it had borrowed shares equivalent to 4.4% of Generali until at least next year's general meeting.
(Reporting by Valentina Za; Editing by Alex Richardson)