Georgia Power flagged new delays at its Plant Vogtle nuclear expansion, a prospect that could lead to more cost overruns paid for by millions of Georgians.

It’s the fourth such announcement the company has made just in the last six months about the troubled construction project, described as the largest in state history.

The delay of another three months is primarily tied to “the need for additional time to address continued construction challenges and to allow for the comprehensive testing necessary to ensure quality and safety standards are fully met,” Georgia Power said in a press release Thursday.

The project near Augusta had earlier faced quality control problems. But in April, Tom Fanning, the chief executive officer of Georgia Power parent Southern Co., had told the investment community, “I think we have resolved the delays, and we think we have a clearer path.”

The expansion is already years behind schedule and billions of dollars over budget. At the start of the year Georgia Power was slated to have its first of two new reactors go into full operation this November. It has repeatedly pushed back its time frame since then.

Now, the state’s largest electric utility said the first of the reactors won’t be in full operation until the third quarter of next year. That’s three months later than it had announced in late July. And the company now says the second new reactor also will be delayed another three months, to the second quarter of 2023.

“As we’ve said from the beginning of this project, we are going to build these units the right way, without compromising safety and quality to achieve a schedule deadline,” Chris Womack, Georgia Power’s chief executive officer, said in a press release. “We have endured and overcome some extraordinary circumstances building the first new nuclear units in the U.S. in more than 30 years.”

The latest announcement comes as elected members of the Georgia Public Service Commission are considering how much of the first wave of the Vogtle project’s construction costs should be added to the bills of Georgia Power customers. A territorial monopoly, Georgia Power needs sign off from the state regulators before increasing charges.

The PSC is expected to vote on the matter early next month.

For years, Georgia Power’s customers have been paying Vogtle financing costs and a portion of the company’s profits on the massive nuclear power project. Cumulatively, those payments alone will have topped $850 for the typical residential customer by the time the first of the new reactors is slated to begin producing electricity.

A proposed agreement struck earlier this month by the company and the PSC’s public interest advocacy staff would add $2.1 billion of Vogtle construction expenses into the company’s rate base once the first reactor is completed. Under that scenario, that could add up to around $3.78 a month in typical residential bills, according to Georgia Power.

Additional Vogtle construction costs could be added to customers’ bills once the second of the new units is completed.

Georgia Power customers aren’t the only ratepayers likely to face higher charges because of Vogtle. Most electric cooperatives and city utilities in Georgia are financially tied to the project.

The first new reactor was originally slated to be in operation in the spring of 2016, followed by a second one a year later.

The expansion eventually could power more than 500,000 homes and businesses. Supporters say the project will offer a long-term source of electricity that doesn’t have carbon emissions and is more reliable than solar and wind power.

Some environmental and consumer groups warned the PSC before it approved the project that costs were likely to soar over those of other sources of energy, and that nuclear energy carries big safety and environmental risks, including long-lasting radioactive waste.