"The Agency has serious doubts as to whether the procedure is conducted for the long-term benefit of the bank and its shareholders, in particular the majority shareholder, the Republic of Slovenia," AUKN said in a letter to NLB released on Wednesday.

NLB and NKBM are a part of a consortium selling a majority stake in retailer Mercator. The sellers are currently in exclusive talks with Croatian retail and food group Agrokor.

The letter comes after daily Dnevnik obtained a draft sales agreement suggesting that the sellers were willing to give Agrokor a year to pay up, while failing to include safeguards protecting Slovenian suppliers and employees.

Sentiment against Agrokor has been building up following earlier revelations that the Croatian company would carry out a leveraged takeover, raising fears that it would squeeze money out of Mercator to pay for the acquisition.

AUKN told the NLB and NKBM managements and supervisors that all commitments made by the buyer must be "insured with appropriate instruments, in particular sufficient financial guarantees."

It expects the banks to analyse the transaction in terms of financial stability and risk management, to transparently reveal the financing of the takeover, and to evaluate the overall effects of the sale succeeding or failing.