Its management, led by Chairman Alessandro Profumo and CEO Fabrizio Viola, wanted to launch the rights issue in January but were defeated at a shareholder meeting on Saturday after the bank's top investor voted against their proposal.
The unprecedented clash between the lender's executives and its main shareholder - a charitable banking foundation with close links to Siena politicians - cast a pall over a tough restructuring meant to revive its fortunes.
Sources close to the matter said Profumo, a strong-willed and internationally respected banker who was formerly the chief of UniCredit, might resign.
Profumo had already secured a pool of banks ready to guarantee the rights issue, but only if it was carried out by end-January.
He said delaying it would make fundraising harder because it would likely coincide with a string of cash calls by other Italian and European lenders, and could precipitate the Tuscan bank's nationalization.
"Today we are certain we can pull it off. If we delay it, we enter in an area of uncertainty," Profumo said.
But the cash-strapped Monte dei Paschi foundation - whose stake is big enough to veto any unwanted decision - forced a postponement to win more time to sell down its 33.5 percent holding and repay its own debts.
An aide described the 56-year-old Profumo, who quit UniCredit in 2010 after clashing with that bank's foundation shareholders and joined Monte dei Paschi in April 2012, as "very annoyed".
He said on Saturday a board meeting was already scheduled for January and he would make up his mind on whether to step down then.
"These are decisions one takes in cold blood and in the right place," he said.
Italian newspapers said former European Central Bank executive board member Lorenzo Bini Smaghi and Carlo Salvatori, who sits on the board of German insurer Allianz, were among possible candidates to replace him if he steps down.
Antonella Mansi, a feisty 39-year-old businesswoman recently appointed head of the Monte dei Paschi foundation, said her insistence on a cash call delay did not amount to a no-confidence vote in the bank's management.
Mansi said that carrying out the capital increase in January would massively dilute the foundation's holding, leaving it with virtually nothing to sell to reimburse debts of 340 million euros.
"We have a precise duty to ensure (the foundation's) survival. You can't ask us to let it collapse," she said.
Analysts however said a delay, and the possibility of Profumo resigning, might undermine the whole rescue of Monte dei Paschi.
"It's important to carry out the capital increase as early as possible," said Roberto Lottici, fund manager at Ifigest. "The risk is that the bank finds itself rushing into a cash call later at a lower price than what it could achieve now."
The rights issue, along with a painful restructuring plan, is among the conditions the European Commission imposed before giving its green light to the state aid for Monte dei Paschi.
But in Siena, where the bank is known as "Daddy Monte" and is the biggest employer, fears that the cash call might sever the umbilical cord between the lender and the city run high.
Siena mayor Bruno Valentini, whose city council is the top stakeholder in the Monte dei Paschi foundation, said on Friday postponing the rights issue might help keep the bank in Italian hands.
"We cannot let the third biggest bank in this country fall prey to foreign interests," he said. "Monte dei Paschi is not just an issue in Siena, it is a big national issue."
Under the agreement with Brussels, if Monte dei Paschi cannot complete the capital increase by the end of 2014 the Treasury would convert the bonds it bought from the bank into shares, effectively nationalising the bank.
The bank, which is cutting 8,000 jobs and shutting 550 branches, said a delay in the cash call would add at least 120 million euros of costs from interest payments on the state debt.