The Financial Regulator is investigating a multi-billion dollar deposit that was placed in Anglo Irish Bank last year by rival lender Irish Life & Permanent.
The deposit, which reportedly amounted to €4bn, was made just before the end of Anglo's financial year and only hours after the Government announced its wide-ranging bank guarantee.
It was reportedly withdrawn around a week to 10 days later.
The deposit by IL&P allowed Anglo to show a higher level of deposits on its books on the day the auditors assessed its accounts.
The Financial Regulator is already investigating how former Anglo chairman Sean Fitzpatrick hid up to €122m in private loans with the bank by temporarily moving them to another institution at the end of each financial year.