Feb 16 (Reuters) - U.S. energy company Southern Co on Thursday delayed the timing and boosted cost estimates for its Georgia Power utility's share of two nuclear reactors being built in Georgia.

The Vogtle plant reactors in Burke County, Georgia, already billions of dollars over budget and years behind schedule, are the only nuclear power units under construction in the United States.

In an investor presentation, Southern forecast Georgia Power costs would rise to $10.593 billion, up from a prior forecast of $10.383 billion in its third quarter results in October.

Southern also pushed back the in service dates for the new reactors to May or June of 2023 for Unit 3 and late in the fourth quarter of 2023 to the end of the first quarter of 2024 for Unit 4.

"After careful consideration and given our experience on Unit 3 and the degree of critical work ahead of us, we are further risk adjusting our Unit 4 schedule," Southern Co Chief Executive Thomas Fanning said on a call with investors.

Fanning, who will be succeeded by Georgia Power's CEO Chris Womack in the coming months, said Unit 3 required additional fixes to pipes, a valve and flow through the reactor's cooling pumps. He added that the company expects testing on Unit 4 to reveal more needed fixes.

"We're just trying to get anything we can see right now," Fanning said.

In January, Southern said in a filing with the U.S. Securities Exchange Commission that it expected Unit 3 to enter service during April 2023. In its third quarter earnings, the company said it expected Unit 3 to enter service in the first quarter of 2023 and Unit 4 in the fourth quarter of 2023.

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 for all owners and enter service in 2016 and 2017.

Some analysts have estimated total costs, including financing, have ballooned to more than $30 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.

The Vogtle owners include Georgia Power (45.7%), Oglethorpe Power Corp (30%), Municipal Electric Authority of Georgia (22.7%) and Dalton Utilities (1.6%).

Oglethorpe and Dalton have said they wanted to freeze their spending on the project.

(Reporting by Scott DiSavino and Laila Kearney; Editing by David Gregorio)