The European pressurised water reactor (EPR), already more than 12 years behind schedule, was supposed to come fully online in December, but the start was pushed back several times during its testing phase.

"Test production has been completed and regular electricity production started today," TVO said. "From now on, about 30 percent of Finnish electricity is produced in Olkiluoto," which already had two reactors.

Finland had been expecting to rely on the new reactor for its electricity needs this winter, given fears of energy shortages after Russia, a major supplier to Europe, invaded Ukraine.

The most powerful nuclear reactor in Europe, with a capacity of generating 1,600 megawatts, Olkiluoto 3 reached full power in late September for the first time since construction began in 2005.

The French-developed EPR was designed to relaunch nuclear power in Europe after the Chernobyl catastrophe of 1986, and was touted as offering higher power and better safety.

But EPR projects in Finland, France and Britain have been plagued by delays and cost overruns.