July 29 (Reuters) - Southern Co's Georgia Power unit on Thursday said it delayed the start dates for two reactors under construction at the Vogtle nuclear plant in Georgia and that its share of the cost would rise by $460 million to an estimated $9.2 billion.

Georgia Power, which owns 45.7% of Vogtle, said Unit 3 was now on track to enter service in the second quarter of 2022 and Unit 4 in the first quarter of 2023.

Previously, the company said both reactors, which are already billions of dollars over budget and years behind schedule, were expected to enter service next year with Unit 3 in the first quarter of 2022 and Unit 4 by its regulatory-approved in service date of November 2022.

The Vogtle reactors are the only nuclear units under construction in the United States and will be the country's first new reactors to enter service since 2016, when Tennessee Valley Authority finished Watts Bar 2 after 40 years of starts and stops.

Those delays and cost overruns could make it difficult for new nuclear to play much of a role in President Joe Biden's goal of getting all U.S. power from non-carbon-emitting sources, like nuclear and renewables, by 2035.

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

Some analysts estimate total costs have ballooned to more than $27 billion following delays related to the pandemic, the nuclear accident at Japan's Fukushima plant in 2011 and the 2017 bankruptcy of Westinghouse, the project's former contractor.

Other Vogtle owners include Oglethorpe Power Corp (30%), Municipal Electric Authority of Georgia (22.7%) and Dalton Utilities (1.6%).

In addition, to construction costs, Southern has estimated its financing costs at around $3.0 billion.

(Reporting by Scott DiSavino; editing by Barbara Lewis)