Peconic Bay Medical Center's board of directors has unanimously voted to join the North Shore-LIJ Health System.

The decision by the 25-member board of the Riverhead hospital Thursday night means the end of a 10-year relationship with Stony Brook University Hospital. It also expands North Shore-LIJ's reach to the East End, making Peconic the 20th hospital in the health system, one of the biggest in the country.

And it means the end of the East End Health Alliance formed in 2008, which included the three East End hospitals, Peconic, Southampton Hospital and Eastern Long Island Hospital in Greenport.

That process started in October 2012 when Southampton announced it planned to merge with Stony Brook.

Paul Connor, CEO of Eastern Long Island, said his hospital's board is also in talks with Stony Brook and North Shore-LIJ and is expected to vote on a merger at its May 28 meeting.

State Sen. Kenneth LaValle (R-Port Jefferson), a staunch Stony Brook supporter, vowed to fight Peconic's decision.

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"It was a bad decision in terms of the health care for the people I represent," he said. "I'm not going to cede the health care for people in Suffolk County to Nassau County."

LaValle said he was going to explore antitrust concerns and the fact that North Shore-LIJ has its own insurance company. The health system must notify the U.S. Department of Justice about the prospective affiliation; the department has 30 days to respond if there are any antitrust concerns.

Spokesman Terry Lynam said North Shore-LIJ owns and operates three Suffolk hospitals. "We've had a strong presence in Suffolk County for more than 20 years," he said.

Riverhead Town Supervisor Sean Walter said he was "100 percent in favor" of the agreement because he believed North Shore-LIJ will expand health services in the region.

"I don't want to take anything away from Stony Brook," he said, but Peconic acted as a "feeder hospital" to Stony Brook. "With North Shore-LIJ, they want to set up roots on the East End and they will set us up as a regional health facility."

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In a statement, Stony Brook said: "For nearly a decade, Stony Brook University Hospital and the physicians of Stony Brook Medicine have formed close relationships with physicians, allied health care providers, emergency medical service professionals, hospitals and members of the community on the East End of Long Island. Stony Brook will continue to work with and support local health care providers to expand the scope of services on the East End and to enhance their access to premier care in the region."

Andrew Mitchell, Peconic's CEO, said the vote culminated discussions about the future of the 200-bed hospital that started soon after Southampton announced its merger with Stony Brook.

Mitchell said the board was especially interested in establishing advanced cardiac services -- such as catheterization labs -- ambulatory services, cancer treatment centers and a trauma center for the East End.

"The agreement is very specific and clear in its intent to elevate programs and services that are truly needed on the East End," Mitchell said.

No money exchanges hands in the agreement. But Mitchell said North Shore-LIJ "made a commitment of sufficient capital and advanced programs to invest in the hospital." He would not provide a figure.

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The agreement with Stony Brook, which began in 2006 and ends next year, included using Stony Brook emergency doctors and radiologists, Mitchell said.

Peconic was an affiliate member of North Shore-LIJ between 2000 and 2006, and North Shore-LIJ chief executive Michael Dowling said he was "thrilled to be renewing" the relationship.