FALMOUTH, Mass. — The U.S. Coast Guard concluded yesterday that it would be too risky to allow liquefied natural gas tankers to travel through Mount Hope Bay and the Taunton River to a proposed LNG terminal in Fall River, a decision that might present an insurmountable roadblock for the project developers.

“After a careful analysis ... I find that the only reasonable conclusion is that the navigation safety risks associated with the vessels of the proposed dimensions and transit frequencies are unacceptably high,” said Roy A. Nash, the Coast Guard’s captain of the Port for Southeastern New England, at a news conference at the Coast Guard station at Woods Hole.

In a letter sent yesterday to the project developer, Weaver’s Cove Energy LLC, Nash said the area of most concern is the waterway from Prudence Island to the proposed site in Fall River. That waterway, Nash wrote to the company, “is unsuitable from a navigational safety perspective for the type, size, and frequency of LNG marine traffic associated with your proposal.”

As elected officials in Rhode Island and Massachusetts yesterday lauded the Coast Guard’s decision, Weaver’s Cove Energy LLC, said it would appeal, and questioned the facts Nash used to reach his conclusion.

“The decision disregards critical facts in the record and introduces both new data and new concerns on which Weaver’s Cover Energy was not provided an opportunity to comment,” the company said in a statement. “The Captain’s decision lacks the necessary factual support and we intend to appeal.”

“We haven’t gone through the document thoroughly,” Weaver’s Cove spokeswoman Marcia MacClary said in a brief telephone interview yesterday. “It will take us some time to review it. The important thing is we are moving forward.”

The company is owned by Hess LNG LLC, which is a joint venture owned equally by Poten & Partners and Amerada Hess Corp.

In an interview, Nash said there are several levels of appeal within the Coast Guard that Weaver’s Cove could pursue. First, the company must appeal to Nash directly, within 30 days. If Nash chooses not to reverse his decision, he would forward the appeal up the chain of command, to the First Coast Guard District in Boston. The final appeal within the agency lies with the Coast Guard headquarters in Washington, D.C., he said.

Weaver’s Cove first proposed building the LNG terminal in 2003. The project has been widely opposed by elected officials in Massachusetts and Rhode Island, as well as citizens groups and organizations in both states. Rhode Island was drawn into the debate because the LNG tankers that would supply the terminal would traverse a 26-mile route through Mount Hope Bay. Opponents of the project say that the 725-foot LNG tankers that would make up to 130 trips a year would disrupt other uses of Narragansett Bay, and would create the risk of a disaster in the event of an accident or terrorist attack.

Despite that opposition, in 2005, the Federal Energy Regulatory Commission, the federal agency that issues permits for onshore LNG facilities, gave the Weaver’s Cove proposal its conditional approval.

Among those conditions was to gain additional approvals from several local and federal agencies, including the Coast Guard.

That agency first expressed concerns about LNG transits in March of last year, when Nash said that for tankers to safely pass through the old and new Brightman Street bridges over the Taunton River would require “extraordinary maneuvers.”

The bridges — the new one is still under construction — are only 1,100 feet apart, and their openings are not aligned, creating a navigational maze for large tankers.

In May, Nash issued his preliminary finding, which was skeptical of whether LNG tankers could safely traverse that maze. The waterway, he wrote, “may not be suitable for the type and frequency of LNG marine traffic” that the company has proposed.

Yesterday’s decision comes three weeks after an announcement by National Grid that it would not continue to appeal a decision by federal regulators that rejected the company’s proposal to expand its LNG facility in Providence.

Lawmakers, who have almost unanimously opposed both projects, hailed the decision.

“The Coast Guard made the right decision,” U.S. Sen. Jack Reed said in a statement. “From a public safety and environmental standpoint, the Weaver’s Cove LNG project posed too many risks and would have placed a tremendous burden on local law enforcement and taxpayers.”

“The Coast Guard’s decision is good news in our efforts to stop this project and protect the environmental health of the Bay and the safety of thousands of Rhode Islanders,” U.S. Sen. Sheldon Whitehouse said in a statement.

“I applaud the Coast Guard for taking all aspects of the proposal into consideration and ruling on the side of safety,” said U.S. Rep. Patrick Kennedy.

“Our years of fighting the siting of LNG in a densely populated area have finally paid off,” said Rhode Island Attorney General Patrick C. Lynch, one of the key opponents of the project.

“The Coast Guard letter confirms what we have been saying all along and we hope this will not be overruled,” said U.S. Rep. Barney Frank, D-Massachusetts. “It is our hope that safety will be allowed to win out over political considerations.”

“I’m very pleased that the U.S. Coast Guard today agreed with my assessment that transporting LNG through Rhode Island’s waterways poses too many environmental and security risks to Narragansett Bay and to nearby residents,” said Governor Carcieri. “This is great news for Rhode Island and for everyone who treasures the Bay.”

Both Carcieri and Lynch called on Weaver’s Cove to end its quest to build an LNG facility in Fall River.

“I now formally call upon Hess LNG LLC to cease proceeding any further,” Lynch said. “I hope it recognizes the futility of pressing on.”

tbarmann@projo.com