OSLO, Dec 28 (Reuters) - Finland's much-delayed Olkiluoto 3 (OL3) nuclear reactor resumed test production of electricity early on Wednesday after a two-month outage, operator TVO said on its web site, in line with a schedule published late on Tuesday.

In a blow to Finland's energy security this winter, the operator in October said cracks were found in the reactor's four feedwater pumps after test production, further delaying a startup that was originally scheduled for as long ago as 2009.

While the damaged pumps posed no security risk, they are essential to the reactor's power output, TVO has said.

Test output had been scheduled to restart on Dec. 27 but was later postponed by one day.

TVO predicted OL3 will hit full capacity on Wednesday followed by tests in the coming week at different output levels, before a month-long shutdown and additional testing ahead of a scheduled March 8 start of permanent production.

Finland has told its citizens to prepare for possible blackouts this winter as Europe seeks to curb energy usage, grappling with reduced Russian gas supplies amid the Ukraine war.

During testing in September, OL3's output for the first time hit full capacity of 1,600 megawatts, making it the most powerful electricity production facility in Europe and the third-most powerful globally, according to TVO.

It is Finland's first new nuclear plant in more than four decades and Europe's first in almost 15 years, with a capacity equal to about 14% of total Finnish electricity consumption, TVO has said.

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.

Under construction since 2005, OL3 has faced several technical mishaps, which sparked costly delays and a lengthy legal battle. (Reporting by Stine Jacobsen and Terje Solsvik; Editing by Kenneth Maxwell)