A construction and newspaper magnate, Caltagirone is Generali's second-biggest investor with an 8.04% stake. He has opposed the reappointment of Donnet, who is backed by Generali's leading investor Mediobanca and a majority of board members.

Generali said in a statement Caltagirone felt thwarted by the board's working methods, particularly its handling of a strategic plan, treatment of privileged information and relations with the media and other shareholders.

Generali Group's Chairman Gabriele Galateri di Genola rejected the reasons given by Caltagirone and said that the company has always conducted its business with absolute transparency and rigorous fairness.

Caltagirone began his offensive in April when he snubbed a general meeting to approve Generali's results. In September, he struck an accord with Leonardo Del Vecchio, the eyewear billionaire who is the third-largest investor in Generali, to consult on decisions about the company.

Smaller Generali investor CRT later joined the consultation pact which represents a combined stake of more than 16% after Caltagirone and Del Vecchio upped their holdings in recent months.

To counter their influence, Mediobanca has borrowed shares to reach a 17% voting stake. The two fronts will face off at an April general meeting to appoint a new board.

Caltagirone and Del Vecchio are ready to back an alternative CEO candidate and present their own slate of candidates for the board, people close to the matter have told Reuters.

(Reporting by Shivani Tanna and Maria Ponnezhath in Bengaluru and Valentina Za in Italy; Editing by Cynthia Osterman)