The Bank of Italy has weighed into a financial scandal engulfing Italy’s oldest bank, Monte dei Paschi di Siena, by alleging that the lender hid documents from regulators about derivatives transactions that may cause it to restate hundreds of millions of losses.

The disclosures, confirmed by the bank’s new management led by Alessandro Profumo, former chief executive of UniCredit, have inflamed the Italian debate just a month away from a national elections where anti-banker and anti-business sentiment is expected to hand significant votes to populist parties.

Shares in Monte dei Paschi, which is seeking a second bailout from the Italian state in four years to remain viable, have plunged in the past five days since revelations in the media about the use of derivatives transactions to hedge against the bank’s portfolio of €24bn of Italian sovereign debt.

In an unprecedented statement released on Wednesday, the Bank of Italy said the “true nature” of some of the derivatives transactions emerged only recently, “following the discovery of documents kept hidden from the supervisory authority and brought to light by the new management of MPS”.

“The transactions are now being examined by both the supervisory and the judicial authorities, in close collaboration,” the bank said.

Under Italian law, if the administrators and managers of an Italian bank keep a document hidden from inspectors it can be considered an obstruction of supervisory activity and a criminal offence.

Giuseppe Mussari, who was chairman at the Tuscan bank at the time the derivatives deals were agreed, resigned from his role as head of the Italian banking association on Tuesday. Mr Mussari said he had done nothing wrong but he did not want to drag the banking lobby into a public debate.

Monte dei Paschi, which has requested a €3.9bn bailout from the Italian state, has said it is reviewing three derivatives transactions called “Alexandria”, “Santorini” and “Nota Italia”. Management said it had discovered these documents only in October and would present its findings to the board next month.

The 500-year-old Tuscan bank, which considers itself to be the world’s oldest, has said aid already requested from the government was sufficient to absorb the impact of the transactions. It added it may decide to renegotiate the deals.

Monte dei Paschi’s former management led by Mr Mussari and the former chief executive Antonio Vigni exited the bank in April last year after pressure from the Bank of Italy and the lender’s main shareholder, a charitable foundation in Siena which owns a third of the bank.

Nonetheless, senior bankers have also questioned why the central bank, then led by Mario Draghi, allowed Monte dei Paschi to undertake a buyout of local lender Antonveneta in 2007, a costly deal that drastically undermined its capital strength.

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