Gov. Andrew M. Cuomo and dozens of elected officials on Thursday signed a letter to President Barack Obama saying New York will sue the federal government if the U.S. Environmental Protection Agency proceeds with a plan to add one or two open-water dump sites in Long Island Sound and allow dredged sludge to be disposed of there.

At Sunken Meadow State Park, with the Sound’s waters glistening in the background, the governor, elected officials from multiple levels of government and environmental activists gathered to speak as one in condemning the EPA plan that would allow dredged soil and sediment to be disposed of in the waters near Fishers Island.

“If the federal government goes ahead with dumping additional dredged material, we will take every action that we can, every resource that we can,” Cuomo said. “And if that includes legal action to stop the federal government, that is exactly what we will do, because this cannot happen, period, my friends.”

At issue is a plan by the U.S. Army Corps of Engineers which, subject to EPA approval, would allow the dredged material to be dumped in the eastern region of the Sound.

The Army Corps, which has rejected alternative disposal methods as too costly, has said most of the dredged soil and sediment is safe for open-water dumping. The EPA agrees.

The opposition cuts across political party lines and has united officials at the town, county, state and federal levels.

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Among the guests who sat in white folding chairs and listened to the governor and other speakers were Rep. Lee Zeldin (R-Shirley), state Sens. John Flanagan (R-East Northport) and Kenneth LaValle (R-Port Jefferson), Assemb. Steve Englebright (D-Setauket), Suffolk County Executive Steve Bellone, and Adrienne Esposito, executive director of the Citizens Campaign for the Environment.

Cuomo, Flanagan and others said that with advanced technological improvements, the federal government should look for other ways to get rid of dredged sludge rather than continue to rely on open-water disposal.

“Why are we dumping anything in Long Island Sound,” Flanagan asked.

EPA spokesman Dave Deegan, noting that the agency has not reached a final decision, said it is “confident that its proposal to designate an open-water disposal site in eastern Long Island Sound strikes an appropriate balance.”

Since the 1970s, he said, the EPA has worked with the states and other federal agencies to apply stringent sediment testing requirements to all dredging projects and prevent dredged material that is contaminated from being placed at open-water sites.

The Army Corps is overseeing 52 navigation projects in various waterways — 31 in Connecticut, 17 in New York and four in Rhode Island. Earlier this year, it estimated that material dredged during work on various projects could produce as much as 53 million cubic yards of sludge over the next three decades. Most of the soil and sediment come from Connecticut.

State and local environmental officials do not dispute that harbors and ports need to be dredged to allow boats, cargo ships and passenger liners to pass safely, but they disagree on where to dispose of the material.

“Environmental and civic groups have consistently and rigorously opposed the dredge dumping scheme,” Esposito said Thursday in a statement. “The public loves this water body and has always considered the Long Island Sound to be an extension of our home . . . We expect the EPA to protect the Sound, not pollute the Sound.”

Soil and sediment pulled from the bottom of rivers, lakes and harbors can contain mercury, lead and pesticides, and is harmful to marine life, said critics, who have urged federal officials to repurpose the material and put it to use.

Since the 1980s, dredged material has been dumped at four open-water disposal sites in the Long Island Sound. The EPA in July said that, starting this month, it will allow the sludge to be disposed of at two sites in the western and central portions of the Sound over the next three decades.

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However, the EPA said those sites don’t have the capacity to handle the anticipated volume over the next 30 years. So the agency proposed adding up to two more dump sites in the Sound’s eastern region.

Among those are the locations called Cornfield Shoals and New London, both near Fishers Island. Permits for those two areas expire Dec. 23.

The EPA wants to keep the western half of the New London site open and expand it to include an adjacent portion to its west. This location, named Eastern Long Island Sound, is EPA’s preferred disposal site, although the agency is considering several options.

New York’s Department of State, which governs the coast, and the Department of Environmental Conservation disagree with the EPA’s conclusion. In a July 18 letter, the state agencies said additional disposal sites are not needed.

The EPA is expected to issue a final decision in the fall.