(Adds details, Slovenske statement)
PRAGUE, May 13 (Reuters) - Slovakia's nuclear regulator has given approval for the newly built Unit 3 at the Mochovce nuclear plant to be brought online, the watchdog said on its website on Thursday.
The plant owned by Slovenske Elektrarne, which is in turn owned by the Slovak government, Italy's Enel and Czech group EPH, began construction of its third and fourth units in the late 1980s during the Communist era only for work to be halted in the 1990s.
Construction of the new units, which are based on the Russian VVER design with control systems from Siemens and Areva, resumed more than a decade ago but has suffered repeated delays and cost overruns.
Estimates from 2019 put the cost at nearly 5.7 billion euros ($6.89 billion).
The regulator said fuel could be loaded into the reactor only after the decision takes legal effect, which can take months if involved parties or members of the public raise protests.
Slovenske confirmed the permission but company officials were not immediately available for further comment.
The company's website said that Unit 3 was 99.95% complete in March, with Unit 4 at 88%.
The two new units at Mochovce are expected to cover about a quarter of Slovak energy demands.
Commissioning of Unit 3 will allow the country reach energy self-sufficiency for the first time since 2007, Slovenske said in Thursday's statement.
Slovenske, the country's dominant power company, operates two nuclear units at Mochovce, one nuclear unit at Jaslovske Bohunice, plus 35 hydropower plants and two coal plants. ($1 = 0.8277 euros) (Reporting by Jan Lopatka Editing by David Goodman)