The New Jersey agency charged with protecting the Pinelands, a vast and fragile expanse of sand pines, gnarled oaks and river deltas, narrowly defeated a proposal on Friday to run a 22-mile natural gas pipeline through it.

The decision dealt a defeat to Gov. Chris Christie, whose administration vigorously lobbied for the pipeline, saying it was an important economic development tool for southern New Jersey. The Pinelands sit atop a shallow, trillions-of-gallons-large aquifer that serves millions of residents. There are 17 species of plants that are found there but nowhere else, said Jeff Tittel, director of the New Jersey Sierra Club, an environmental group.

Four former governors, two Republican and two Democrat, wrote a letter opposing the project.

The New Jersey Pinelands Commission rejected the proposal by South Jersey Gas, which had teamed up with the state Board of Public Utilities to connect a pipeline to the B.L. England power plant in Cape May County, The Associated Press reported on Friday. The plant is switching to natural gas from coal, The A.P. said.

“I hurt my back doing cartwheels,” in response to the vote, said Mr. Tittel on Saturday. “This is the biggest environmental win we’ve had” during the Christie administration, he said.