HELSINKI, Dec 9 (Reuters) - Finland faces a greater risk of power outages in coming months because of another delay in starting up the new Olkiluoto 3 (OL3) nuclear reactor, national grid operator Fingrid said on Friday.

Fingrid and the state energy authority have told citizens and companies to prepare for possible blackouts, particularly if OL3 does not prove reliable, as countries across Europe seek to curb energy usage, grappling with reduced Russian gas and other energy supplies because of the Ukraine war.

"There will be less domestic production capacity available than was previously estimated which highlights the importance of saving electricity and timing its usage to safeguard its sufficiency," Fingrid said in a statement.

Finns cut power consumption by 7% year on year in October, but consumption usually peaks during cold winter months in January and February as they turn up the heating.

The OL3 plant 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 Finnish electricity consumption, TVO has said.

In October, the operator said cracks were found in the OL3 reactor's four feedwater pumps after test production, further delaying startup originally planned to be in 2009.

Test production has been postponed until Dec. 25 at the earliest, from Dec. 11, TVO said, while full production will begin on Feb. 6 at the earliest compared with Jan. 22.

"The investigation into the damage in Olkiluoto 3's feedwater pumps has proceeded into its final stages. Once the investigation is complete, a decision will be made on the startup of the plant unit," TVO said in a statement.

Adding to Finland's concerns, neighbouring Sweden, a major electricity exporter, switched off its Oskarshamn 3 nuclear reactor for maintenance from Dec. 9-18 and its Ringhals 4 reactor is offline until late February.

"Weather during the upcoming winter and availability of power imports from southern Sweden are other significant elements of uncertainty in terms of power sufficiency," Fingrid manager Tuomas Rauhala 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 technical mishaps, which sparked costly delays and a lengthy legal battle. (Reporting by Anne Kauranen in Helsinki and Terje Solsvik in Oslo, editing by Stine Jacobsen and Barbara Lewis)