"The plant was disconnected from the grid unplanned around noon as a result of a fault on the turbine side," TVO spokeswoman Johanna Aho told AFP.

The issue concerns the part of the plant where steam is used to produce electricity, not the reactor side.

"The plant is completely safe," Aho said. "But it is not currently generating electricity into the national grid".

The issue arose a day after the plant resumed test production following the completion of automation updates.

The reactor built by the French-led Areva-Siemens consortium on Finland's southwest coast went online in March, 12 years behind schedule.

The start date for OL3's normal production is being closely followed in Finland, where many hope the plant will ease the coming winter's energy challenges.

TVO said it did not yet know the exact cause of the problem but said staff was investigating.

"When we find out, we will know if this is a temporary problem or if something else needs to be fixed," Aho said.

She was unable to say whether the issue would affect the planned launch date.

Olkiluoto 3 is currently due to enter normal operation in December 2022, after the test phase is concluded.

Energy production had been expected to start this summer, with the aim of supplying about 15 percent of Finland's electricity consumption.

But the start date has been pushed back multiple times this year alone.

In April, it was moved to September due to repairs needed on the cooling system.

And in June it was postponed to December, after "foreign material" was observed in the turbine's steam reheater.

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