March 22 (Reuters) - Southern Co said the planned November startup of the third Vogtle nuclear reactor in the U.S. state of Georgia could be delayed for additional construction.

The Atlanta-based company said it "continues to target a November in-service date for Unit 3," but "the schedule is challenged and ... a delay is likely and could add one month or more to the in-service date," according to a filing on Friday.

Southern said "any schedule extension beyond November for Unit 3 is currently estimated to result in additional base capital costs ... of approximately $25 million per month."

Delays and cost overruns in building reactors could make it difficult for new nuclear facilities to play much of a role in the Biden administration's plan to get all of the nation's power from non-carbon-emitting sources such as nuclear and renewables by 2035.

Despite construction delays and cost increases caused by coronavirus-related workforce reductions in 2020, Southern has long maintained it could complete Units 3 and 4 by their regulatory-approved November 2021 and November 2022 in-service dates.

When Georgia utility regulators approved the Vogtle expansion in 2009, the two 1,117-megawatt Westinghouse AP1000 reactors were expected to cost about $14 billion and enter service in 2016 and 2017.

But analysts estimate costs have ballooned to more than $25 billion due to delays related to the nuclear accident at Japan's Fukushima plant in 2011 and the 2017 bankruptcy of Westinghouse, the project's former lead contractor.

Analysts at Mizuho have said Unit 4 will likely not enter service until 2023 and Southern's costs will likely exceed by more than $1 billion the $7.3 billion Georgia regulators have deemed reasonable.

Southern's Georgia Power unit owns 45.7% of the Vogtle expansion and forecast its part of the total capital cost at around $8.7 billion. The company also estimated its construction financing costs at around $3.0 billion.

(Reporting by Scott DiSavino; Editing by Bernadette Baum and Will Dunham)