The announcement comes after a day after unofficial sources indicated the Commission would make the move because it is dissatisfied with the measures Slovenia proposed in exchange for deferring privatisation.

"Slovenia has proposed changes to its commitment to sell 75% of its shares in NLB and has missed a deadline under its existing commitment. It is the responsibility of member states to comply with their commitments," Commissioner Margrethe Vestager, who is in charge of competition policy, was quoted as saying.

"We have doubts that Slovenia's new proposal is equivalent to its original commitment ... We have to ensure equal treatment of all EU countries. We will continue to work constructively with the Slovenian authorities over the course of this investigation."

At the end of December, the Slovenian government proposed that the start of NLB privatisation be postponed until late 2018 and a blind trustee appointed in the meantime.

This was the country's latest proposal in efforts to defer the sale of the bank, which had been promised in exchange for state aid in 2013. It entails the start of privatisation after the 2018 general election and its completion in 2019.

Finance Minister Mateja Vraničar Erman said yesterday that Slovenia would study the Commission's motion and respond within the one-month deadline. The minister also expects the talks with the Commission to continue during the one-month period.

This was also echoed by the Finance Ministry, which said in response to today's announcement it expected "intensive talks" with the commission during the one-month period.

It also said it was "important that the initiation of the in-depth investigation does not prejudge its outcome and that new opportunities to find an appropriate solution are created."

However, any additional changes to the catalogue of commitments will have to be cleared by the government.