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 the plant down in the absence of assistance from the state legislature.

Three Mile Island had a license to continue operations until 2034.

"Today is a difficult day for our employees, who were hopeful that state policymakers would support valuing carbon-free nuclear energy the same way they value other forms of clean energy," said Bryan Hanson, another Exelon senior vice president.

Nuclear facilities have been under pressure for years from competition from electricity generated from other cheaper sources such as natural gas.

The March 28, 1979 partial meltdown at Three Mile Island remains the worst nuclear accident in US history although it pales in comparison to the disasters at Chernobyl and Fukushima in 1989 and 2011 respectively.

The meltdown registered a five on the International Nuclear Event Scale that peaks at seven, the rate given to the Chernobyl and Fukushima disasters

No one died at Three Mile Island and no one was seriously injured but the accident sparked nationwide concern about the safety of nuclear power and put the brakes on construction of new reactors.