Topeka ? Federal officials have told Westar Energy Inc. that the company violated clean air regulations when it replaced equipment at its largest electric generating plant during the 1990s, Westar disclosed Monday.

The Environmental Protection Agency notified Westar last week that the company had failed to obtain required permits before making changes at the Jeffrey Energy Center outside St. Marys. An EPA spokeswoman said permits helped the agency monitor whether a coal-fired plant, like Jeffrey, was putting too many pollutants into the air.

The EPA has not concluded yet that Jeffrey is violating pollution standards, but Westar acknowledged Monday that it could be forced to upgrade pollution-control equipment — with the cost passed along to its 654,000 customers, which include Lawrence residents.

Westar disclosed the EPA’s notice in a filing Monday with the federal securities regulators, who require companies to report events that could significantly affect their finances. The EPA sent its notice to Westar on Thursday.

Pat Reilly, spokeswoman for the EPA’s enforcement office in Washington, said the agency and the company would try to negotiate a remedy.

“It can go on quite awhile because it involves expense,” Reilly said.

Westar spokeswoman Karla Olsen said the company thought it complied with regulations but was cooperating with the EPA.

“We want to be in compliance, and we’re taking this notification seriously,” Olsen said.

In its filing, Westar said it could not predict the outcome of negotiations, adding, “If the EPA requires us to update emissions controls at Jeffrey Energy Center or take other remedial action, the costs could be material.”

Westar is Kansas’ largest electric utility. It owns 84 percent of the Jeffrey center, with the other 16 percent owned by Aquila Energy Corp.