PwC is being sued for €1bn (£864m) by the administrators of collapsed Irish company Quinn Insurance.
Grant Thornton, the administrators, are alleging that Quinn Insurance could have avoided loses had auditors PwC highlighted some of the problems in the company’s accounts and accused the firm of a negligent audit.
The administrators said they intended to use any damages they win to repay some of Quinn Insurance debts to the Irish state.
PwC denies the accusations and said it stood by the quality of its audit work.
"These proceedings are unjustified and devoid of merit. The affidavit, accompanying the application, contains factual inaccuracies, misrepresentations of the facts and assertions with which we fundamentally disagree," PwC is quoted to have said in a statement to the BBC.
The legal proceedings will be carried out in the Commercial Court in Dublin