The Three Mile Island nuclear plant, site 40 years ago of the worst ever US nuclear accident, is to shut down in September, its owner announced on Wednesday.

Exelon Generation had been seeking subsidies from Pennsylvania lawmakers to keep the money-losing facility open, but the company said that would apparently not happen in time.

"We don't see a path forward for policy changes before the June 1 fuel purchasing deadline for TMI," Kathleen Barron, an Exelon senior vice president, said in a statement.

Exelon said Three Mile Island Generating Station Unit 1, located in Middletown, Pennsylvania, will shut down by September 30.

The energy company had warned two years ago that it may be forced to shut down the plant, which had a license to continue operations until 2034, in the absence of assistance from the state legislature.