The plant, in service since 1983, had been scheduled to be switched off in 2023 but Slovenia and co-owner Croatia decided in 2015 to extend its lifetime for another 20 years.

Activist groups including Greenpeace have demanded its dismantling because of its age and earthquake risks in the region, but the government insists expanding the plant is the best way for it to generate clean energy.

"This is a first step, an important first step, opening the way for the procedures to start," Slovenian Infrastructure Minister Jernej Vrtovec told a news conference in Ljubljana.

But he added that a final decision had yet to be taken on the project.

The energy permit was issued for the construction of a 1,100-megawatt reactor with a lifespan of 60 years, but its location, technology and other conditions have yet to be determined.

The plant covers around 20 percent of Slovenia's electricity needs and 15 percent of Croatia's.