Day has headed the Southern Co subsidiary since 2010 and been at the helm of the Kemper development.

Mississippi Power's vice-president of generation, Tommy Anderson, also resigned recently.

The Kemper giant coal-gasification project is about 95 miles east of Jackson and began construction in 2010. It is yet to be finished and, according to a company statement released last month, the cost of the project has risen from $2.88 billion to approximately $3.42 billion.

The still unfinished “clean coal” plant will convert lignite to gas, which is then piped to a conventional power plant where it is burned to make electricity.

Day will be replaced by G. Edison Holland, previously Southern Co’s general counsel and a former CEO of Savannah Energy.

“Mississippi Power is getting a highly regarded, experienced leader in Ed Holland,” Southern Company chairman, president and chief executive officer Thomas Fanning said in a statement.

“He brings a strong combination of judgment and vision to the role that will propel the company forward. Ed will be engaged in the region and work to strengthen the communities we serve.

Despite speculation that Day has been forced to retire, the company maintains that he resigned, and in an interview with the Wall Street Journal, Holland said it wasn't uncommon for management changes to occur when projects encountered cost overruns but said Day had decided "it was time to move on."

Fanning thanked Day for his time with the company and said Holland would now oversee Kemper.

“Ed Day is a dedicated leader who has had many accomplishments spanning his 30-year career with Southern Company and we wish him well,” Fanning said.

“It’s always tough when lifelong employees choose to move on, but we wish Ed the best in his retirement. He has passed the reins to the right person to lead Mississippi Power.”

As Southern Co’s top lawyer, Holland has led an internal inquiry into the April announcement that Kemper was an additional $540 million over budget and found gaps in Day’s reporting of the overruns.

Mississippi Power has 186,000 customers in 23 counties from Meridian to the Gulf Coast, in southeast Mississippi.