HARRISBURG, Pa. — Three Mile Island, site of the United States’ worst nuclear power accident, will begin a planned shutdown starting June 1 now that it is clear that it will not get a financial rescue from Pennsylvania, its owner said Wednesday.

Exelon Corp.’s statement comes two years after the Chicago-based energy giant threatened to close the money-losing plant without what critics have called a bailout.

The fight over Three Mile Island and Pennsylvania’s four other nuclear power plants invigorated a debate over the “zero carbon emissions” characteristics of nuclear power in the age of global warming and in one of the nation’s largest fossil fuel-producing states.

Three Mile Island’s Unit 1 is licensed to operate through 2034, and shutting it down will cut its life short by 15 years. Power from the plant along the Susquehanna River is expected to be replaced by electricity from coal and natural gas-fired power plants that run below capacity in a saturated market.

It will go offline by Sept. 30, Exelon said.

In a statement, Kathleen Barron, an Exelon senior vice president, said the company doesn’t see “a path forward for policy changes before the June 1 fuel purchasing deadline for TMI.”

A roughly $500 million package for Three Mile Island and Pennsylvania’s four other nuclear power plants has stalled without a vote in the Legislature, and Wednesday was the state Senate’s last scheduled session day of May.

The rescue split the leadership of the state Legislature’s Republican majorities, and Gov. Tom Wolf, a Democrat, never threw his support behind it.

Wolf’s office on Wednesday said he was disappointed at the news, and said it is still essential to maintain and expand Pennsylvania’s “carbon-free energy footprint.”

“I remain hopeful that a consensus on a path forward can be reached in the coming weeks,” Wolf said.

Nuclear power plants around the U.S. have been struggling in recent years to compete with generating stations that burn plentiful and cheap natural gas to produce electricity.

Exelon has won rescues in New Jersey, New York and Illinois, and had allies in organized labor. Next door, in Ohio, lawmakers are embroiled in a debate over rescuing two FirstEnergy Corp. nuclear plants.

But in Pennsylvania, the nuclear power rescue bill drew opposition from the state’s considerable natural gas industry, not to mention industrial users and consumer advocates.

Three Mile Island faced particularly difficult economics because 1979’s terrifying partial meltdown left it with just one reactor.

Decommissioning Unit 1, dismantling its buildings and removing spent fuel could take six decades and cost more than $1 billion, Exelon estimates.

The destroyed Unit 2 is sealed and its twin cooling towers remain standing. Its core was shipped years ago to the U.S. Department of Energy’s Idaho National Laboratory. What is left inside the containment building remains highly radioactive and encased in concrete.