"The investigation into the damage at Olkiluoto 3's feedwater pumps will continue still for some weeks, and its impact on the schedule cannot be estimated," TVO said in a statement.

The reactor, which was first announced in 2003, had been expected to finally come fully online in December following years of delays, but in October TVO reported that during maintenance, the damage had been detected in the feedwater pumps located in Olkiluoto 3's turbine island.

TVO said Monday that the investigation into the cause was continuing and that electricity production would resume on December 11, 2022 at the earliest "and as such regular electricity production starts at the end of January 2023 at the earliest."

Finland is expecting to rely on Olkiluoto's new EPR reactor after a halt in electricity imports from its neighbour Russia.

The new nuclear plant problems came just as winter descended on Europe, with fears of energy shortages after Russia's invasion of Ukraine.

Olkiluoto 3 is the most powerful reactor in Europe and the third in the world.

On September 30 it reached its full capacity for the first time.

At the time, it alone produced about 20 percent of Finland's electricity -- 40 percent when the two existing Olkiluoto 1 and 2 reactors were added.