The lender is in the process of negotiating a €5bn (£4.2bn) rescue package with the Italian government.
Yesterday it requested until mid-January to seal the deal - but a report by Reuters suggested the ECB turned down the request on the grounds the delay would do little to help.
That means it could be bailed-in, with a privately funded rescue plan, as soon as this weekend, the FT reported.
Shares in the lender, the world's oldest, fell 6.3 per cent to €20.48 in lunchtime trading.
The bank's shares tumbled following Italy's referendum last Sunday, and Prime Minister Matteo Renzi's subsequent resignation: Renzi was seen as playing a major part in rescue negotiations.
The Italian government is already Monte dei Paschi's largest shareholder, with a four per cent stake in the business - although Reuters it could recapitalise the bank with as much as €2bn, leaving it with a 40 per cent stake.
In European Banking Authority stress tests in the summer, Monte dei Paschi was the only lender whose capital would be entirely wiped out by a sharp economic downturn.