While it remains uncertain that the claim will be brought, the auditor's dramatic threat – and the size of damages it has suggested – reflects anger in European political circles at the role played by rating agencies in the market turmoil that threatened the eurozone.
The prospect of sovereign downgrades caused waves of selling of government debt from countries such as Italy and Spain, raising their borrowing costs and further damaging their public finances.
The Corte dei Conti is an institution with the constitutional role of "safeguarding public finance and guaranteeing the respect of the jurisdictional system".
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S&P is expected to claim that it has jurisdiction only over government employees and agents andnot over rating agencies.
A spokesman for Moody's also said the allegations were without merit.
Fitch said it would co-operate with the probe. "As we understand the prosecutor's concerns, we believe Fitch at all times acted appropriately and in full compliance with the law."
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A €234bn lawsuit would dwarf all other legal claims brought against the rating agencies over the actions in the run-up to, and during, the financial crisis.
Investors in mortgage-backed derivatives have lodged claims alleging that the agencies exaggerated the creditworthiness of these complex instruments to win business –claims the agencies have denied.
The US government has separately launched a $5bn civil lawsuit against S&P saying it fraudulently inflated ratings on collateralised debt obligations.
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