(Adds details and background)

OSLO, Sept 30 (Reuters) - The output of Finland's Olkiluoto 3 nuclear reactor, Europe's largest, on Friday reached its landmark power level of about 1,600 megawatts for the first time, operator TVO said.

"The plant unit is now the most powerful electricity production facility in Europe and the third-most powerful globally," TVO said in a statement.

The news will be welcome in Finland, where the national grid operator has warned of potential power blackouts in the coming winter if OL3 could not reliably supply electricity.

Imports of power from Russia stopped in May after Russian utility Inter RAO said it had not been paid for the power it sold, increasing the need for OL3's output.

Together with the smaller OL1 and OL2 reactors, each producing close to 900 megawatt, TVO's Olkiluoto facilities accounted for around 40% of Finland's power output early on Friday, the company said.

Under construction since 2005, OL3 was originally meant to start operation in 2009, but has faced several technical mishaps, which sparked costly delays and a lengthy legal battle.

Test production began earlier this year, and regular electricity output at the reactor will start in December, TVO added, in line with its most recent prediction. (Reporting by Terje Solsvik in Oslo and Juby Babu in Bengaluru; Editing by Dhanya Ann Thoppil, Subhranshu Sahu and Sherry Jacob-Phillips)