In a major blow to the future of nuclear power in the United States, two South Carolina utilities said on Monday that they would abandon two unfinished nuclear reactors in the state, putting an end to a project that was once expected to showcase advanced nuclear technology but has since been plagued by delays and cost overruns.

The two reactors, which have cost the utilities roughly $9 billion, remain less than 40 percent built. The cancellation means there are just two new nuclear units being built in the country — both in Georgia — while more than a dozen older nuclear plants are being retired in the face of low natural gas prices.

Originally scheduled to come online by 2018, the V.C. Summer nuclear project in South Carolina had been plagued by disputes with regulators and numerous construction problems. This year, utility officials estimated that the reactors would not begin generating electricity before 2021 and could cost as much as $25 billion — more than twice the initial $11.5 billion estimate.

The utilities also struggled with an energy landscape that had changed dramatically since the large reactors were proposed in 2007. Demand for electricity has plateaued nationwide as a result of major improvements in energy efficiency, weakening the case for massive new power plants. And a glut of cheap natural gas from the hydraulic fracturing boom has given states a low-cost energy alternative.