"Any suspicion connected to my conduct, including any wrongdoing in the Mercator case, is unfounded," he told the STA after he was released from custody.

"I've never owned Mercator stock, directly or indirectly. Nor did I disclose any price sensitive information," he stressed.

Police suspect that Medja, then in his capacity as member of the board of Unicredit banka Slovenija, had organised the purchase of Mercator stock after learning that Croatia's Agrokor would bid to take over Slovenia's leading retailer.

Based on this information, a group of banks started purchasing Mercator stock at prices below the takeover price to make money on the eventual sale to Agrokor, according to the NBI.

The investigation has raised questions about his suitability to head the biggest bank in the country, and it has prompted the NLB supervisory board to hold an emergency meeting on Saturday.

Medja said that NLB customers can rest assured. "NLB is led by a professional and honest management board, our sole goal is the comprehensive transformation of the bank into a sustainably successful institution, which will not be undermined by the attempts of those who have other interests."