Poland's Financial Supervision Authority has said it is investigating the deal by Allied Irish Bank to sell its Polish banking assets to Spain's Santander lender.
The PFSA said "certain doubts" have come up and it is checking whether during the tender process any confidential information was leaked.
On Friday, AIB announced it was selling its Polish operations for €3.1bn.
The PFSA said information about the deal is now circulating on the markets which must be checked.