By Gianluca Semeraro and Elvira Pollina

MILAN (Reuters) - One of Generali's biggest investors is considering challenging the reappointment of longstanding CEO Philippe Donnet in May and is weighing potential alternative candidates to lead Italy's biggest insurer, four sources close to the matter said.

Three years ago Francesco Gaetano Caltagirone, an Italian construction and publishing billionaire with a 6.9% stake in Generali led a failed attempt to replace Donnet, who had backing from the insurer's board and its biggest shareholder Mediobanca.

Caltagirone, as well as the late billionaire Leonardo Del Vecchio, has criticised Donnet, at the helm of the insurer since 2016, for failing to grow Generali sufficiently.

More recently the two investors expressed reservations about a proposed asset management joint venture between Generali and France's BPCE over concerns about the influence the French side could have in the partnership.

Caltagirone has not yet decided on his strategy at a shareholder vote on May 8 to elect a new Generali board and could still opt not to nominate a new CEO candidate, one of the four sources said.

But, he is considering proposing alternative candidates including current Enel CEO Flavio Cattaneo, who in 2022 was appointed as a Generali director among Caltagirone's representatives, three of the sources told Reuters.

Cattaneo is expected to be put forward again by Caltagirone as a director. However, proposing him as a candidate for CEO would be more problematic given his current role.

A spokesperson for Enel said his current mandate at Enel does not expire until 2026, "therefore the Enel CEO is not free to assume other roles".

"This rumour is totally groundless as well as destabilizing, also taking into account that Enel is a listed company," the Enel spokeperson said.

There are similar issues with two other candidates who are under the attention of Caltagirone, the sources said.

Caltagirone has good relations with national postal service Poste Italiane CEO Matteo Del Fante and Rome utility Acea boss Fabrizio Palermo, the sources said, adding he is considering either as a replacement for Donnet.

Representatives for Poste, Acea and Caltagirone all declined to comment.

Donnet has said he is ready to stay on for another term, but, unlike the last time round, he is not being put forward by Generali's board after Italy's government changed corporate rules on succession making it harder for an outgoing board to nominate candidates.

The uncertainty over Generali's leadership comes as top shareholder Mediobanca faces a hostile takeover bid by Monte dei Paschi di Siena (MPS), a state-backed bank in which Caltagirone built a stake since November.

In rejecting the MPS bid on January 28, Mediobanca flagged the cross-shareholdings among MPS, Generali and Mediobanca.

Caltagirone and the heirs of Del Vecchio together own 17% of Generali, 27% of Mediobanca and 15% of MPS.

Three years ago, Caltagirone and Del Vecchio were defeated in a shareholder vote after putting forward former Generali executive Luciano Cirina as their CEO candidate.

In a surprise move that gives CEO Andrea Orcel weight in the potential clash between Mediobanca and Caltagirone, UniCredit has disclosed a 4.1% equity stake in Generali.

(Reporting by Stefano Bernabei, Giuseppe Fonte, Francesca Landini, Elvira Pollina, Gianluca Semeraro; Writing by Valentina Za; Editing by Giselda Vagnoni and Elaine Hardcastle)