COLUMBIA, S.C. (AP) — Environmental groups are urging South Carolina's utility regulators to stop the construction of nuclear reactors already years behind schedule and return billions in cost overruns to consumers.

The Public Service Commission could save utility customers up to $10 billion by "pulling the plug" on South Carolina Electric & Gas' two new nuclear reactors at the V.C. Summer Nuclear Station in Jenkinsville, about 30 miles northwest of Columbia, and ordering at least some prepaid costs refunded. Those are the conclusions of a report funded by the Sierra Club and Friends of the Earth and released Tuesday.

"It's now time for South Carolina to admit failure," said the report's author, Mark Cooper with Vermont Law School's Institute for Energy and the Environment. "It was a mistake. It was an expensive mistake. It can become a catastrophic mistake if we don't stop now."

The report is being submitted to the Public Service Commission ahead of an October hearing.

SCE&G owns 55 percent of the reactors, while state-owned utility Santee Cooper owns the other 45 percent. Since 2009, SCE&G customers have funded the reactors through a series of rate hikes approved by commissioners. Construction now accounts for 18 percent of residential customers' electric bills.

The utility's plans are uncertain. Westinghouse, the contractor building the reactors, filed for bankruptcy protection earlier this year.

SCE&G's parent company, SCANA, has said abandoning the project is an option. SCANA CEO Kevin Marsh told commissioners in April that other options include finishing one or both reactors. Roughly one-third of the project is complete, said SCANA executive Steve Byrne.

SCE&G and Santee Cooper officials are jointly evaluating what's in the best interests of "customers and other stakeholders," SCANA spokesman Patrick Flynn said in an email.

The results will be submitted to state regulators for review and public comment, he said, calling the October hearing requested by environmentalists premature. "Our focus remains on a thorough analysis and evaluation that includes a broad range of considerations," Flynn said.

The first reactor was initially slated to open in March 2017. It was then set to be online in August 2019, with the second reactor following a year later.

A new target date and final cost are unknown. The report contends the project is wasting $120 million per month, and future cost overruns could top $5 billion.

Whether the commission can order the utility to refund customers, and how much, are matters of debate. That could require proof the utility gave regulators faulty information.

Even with no refund from bills already paid, customers will save money if the project's abandoned, Cooper said.

Under his report's worst-case scenario, reactor costs could end up accounting for two-thirds of a residential bill, rising from the current average of $27 to $100 monthly. His best-case scenario maintains current rates.

The potential range "starts at painful and becomes very painful, even if you avoid heart-attack painful," Cooper said.

The environmental groups have opposed the reactors from the start and repeatedly called for the project's cancellation.

"We did tell you so," said environmental attorney Robert Guild.