The former Anglo Irish Bank is taking legal action against Ernst & Young, which audited the bank before the Irish government decided to wind it down.
Scandal-hit Anglo, now known as the Irish Bank Resolution Corp (IBRC), has been under investigation for fraud for the past four years and three former executives, including ex-chief Sean FitzPatrick, face charges next year.
IBRC said the proceedings related to Ernst & Young’s role as auditors before the bank was nationalised in 2009 but would not give any more detail.
A spokesman said: “As this matter is now the subject of litigation, it would be inappropriate for the bank to comment further at this time.”
Anglo, whose collapse is expected to cost the Irish state at least €25 billion (£20.2m), has been investigated for allegedly giving clients loans to buy shares in the bank and for using deposits from another lender to mask large withdrawals.
Ernst & Young, one of the world’s “big four” audit firms, said it would “vigorously” defend itself.
The firm said: “Although Ernst & Young is aware of proceedings issued by IBRC, we have not formerly been served with, nor have we received, a statement of claim setting out the details of IBRC’s claim.
“Without more detail it is difficult for us to comment further. We have consistently said we stand by the quality of our work performed in the Anglo audit and will vigorously defend any such proceedings.”