(Fixes typo in paragraph 4)

By Gianluca Semeraro, Valentina Za and Giuseppe Fonte

MILAN (Reuters) -Mediobanca's decision to delay the shareholder vote on its bid for Banca Generali prevented a likely defeat from shareholders who oppose the deal and have beefed up their holdings in recent weeks, four sources close to the matter said.

On Sunday, Mediobanca said it would delay until September 25 the vote that had been scheduled for Monday. 

The proposed Banca Generali acquisition was announced in April as Mediobanca strived to fend off a takeover bid by state-backed Monte dei Paschi di Siena, or MPS.

MPS is expected to formally launch its hostile offer for Mediobanca, one of many takeover bids reshaping Italian finance, in July. The bid was announced in January but several months are necessary for all the required authorisations.

Were it to succeed, it would cast doubt over whether Mediobanca would be able to pursue the Banca Generali acquisition.

Mediobanca CEO Alberto Nagel had bet on the Banca Generali deal as an alternative to the MPS project, hoping it would also make Mediobanca too big for the smaller rival to swallow.

But Mediobanca shareholders looking to thwart the Banca Generali deal have accumulated an additional combined near 11% stake in the bank with their allies in the last few weeks, one of the sources said. 

That tilted the odds of Monday's vote against Nagel, prompting the board of the bank to opt to delay the shareholder meeting, the sources said, declining to be named because of the sensitivity of the issue.

The deal risked lacking the support of some 40% of Mediobanca's capital, including also shareholders who would have abstained, a fifth source separately said.

JUDICIAL PROBE

Opposition to the deal is led by Francesco Gaetano Caltagirone who has increased his Mediobanca stake to 10% from 7%, and can count on the support of UniCredit CEO Andrea Orcel.

UniCredit, which faces government opposition to its own bid for Banco BPM, held 1.9% of Mediobanca ahead of Monday's vote as part of its equity trading activity. By abstaining, it would have helped the "against" front.

Nagel has long faced opposition by both Caltagirone and Mediobanca's biggest investor, Delfin, the holding company of late Ray-Ban billionaire Leonardo Del Vecchio.

Delfin and Caltagirone became investors in MPS when the Italian Treasury in November sold a stake in the bailed out bank.

That sale is being investigated by Milan prosecutors who are probing how the process was handled.

The sources said the vote postponement had also bought time to see how the criminal investigation unfolded.

While Delfin Chairman Francesco Milleri has welcomed Nagel's efforts to promote change at Mediobanca, Caltagirone has criticised the Banca Generali deal saying he saw no value in it.

The two are also some of the biggest shareholders in Assicurazioni Generali, Italy's largest insurer, which owns private bank Banca Generali.

By using its Generali shares as payment for the Banca Generali deal, Nagel hoped to win over Delfin and Caltagirone who have repeatedly criticised Mediobanca's influence over Generali.

The latest clash was in April when Generali shareholders picked a new board.

At the time, following a last minute U-turn, UniCredit's Orcel sided with Caltagirone after amassing a 6.7% Generali stake.

(Additional reporting by Giulio Piovaccari; editing by Barbara Lewis)