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.

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

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

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

As recently as April, the operator Teollisuuden Voima pushed back the start of energy production from July to September, due to repairs needed on the cooling system.

On Wednesday, TVO said test production had again been delayed after "foreign material" was observed in the turbine's steam reheater.

"Foreign material detached from the steam guide plates was found in the turbine's steam reheater in May 2022, which requires inspection and repair work", TVO said in a statement.

The repair work will last until the end of July, it said.

The test production programme and electricity production can only be continued after the repairs are completed.

"According to information received from the plant supplier, OL3 EPR's regular electricity production is to start in December 2022, instead of the previously announced start in September 2022", it said.