Management had wanted the 3.0-billion-euro ($4.0-billion) capital increase to be completed by January but a shareholder meeting overwhelmingly backed a proposal by the bank's foundation, which owns a 33.5 stake in the lender, to delay the cash call until May or June.
Paying for the capital increase would hit the foundation badly, prompting it to seek more time to allow it to sell its shares to a third party.
The shareholders' meeting in Siena was originally scheduled to be held on Friday but it had to be moved to Saturday because there were not enough shareholders present to meet the required 50.1 percent threshold.
A delay in recapitalisation could be critical as the European Commission has demanded that the operation be completed by the end of 2014 so as to reimburse the state credits the bank received to avoid collapse.
The shareholders' vote will come as a blow to chairman Alessandro Profumo, who had earlier warned that a delay in raising the capital would carry "grave risks", and there have been reports that he and chief executive Fabrizio Viola could resign if they did not get their way.
The head of the bank's foundation, Antonella Mansi, meanwhile had argued that a capital increase in January would "destroy the foundation's wealth".
Speaking to shareholders on Saturday, Mansi said management's proposal had "no chance of being approved".
The Tuscan bank, which has a proud history of independence dating back to 1472, was bailed out by the Italian state in February to the tune of 4 billion euros.
The bank stands to be nationalised if the state credits are not returned in time.