he proposed Millennium Pipeline, the longest natural gas line under review in the nation, has suffered another setback with the announced withdrawal last week of its Canadian backers because of the project's repeated delays in winning approval in the United States.

In a letter to Canadian energy regulators, the sponsors said they had decided to withdraw their application, at least for now, while they update technical information for the Canadian segment of the project and await a decision from United States regulators on the New York end.

That ruling is not expected until later this year now that the Federal Energy Regulatory Commission, which will rule on the Millennium project, has scheduled another public hearing for next month. The hearing is a response to vociferous complaints from Westchester residents.

The hearing will be held on Sept. 4 in Mount Vernon and delay the commission's final environmental report, which was expected this summer and would guide the commission on deciding whether to approve the project.

The 425-mile New York leg of the pipeline, which would extend across southern New York from the international border in Lake Erie to Mount Vernon, was first proposed in December 1997 by the Columbia Gas Transmission Corporation of Fairfax, Va., with an expected completion date of November 1999.

The United States line would join two proposed lines in Canada that would connect with a transcontinental line drawing natural gas from western Canada. That gas, the sponsors said, would help meet a projected surge in demand in New York, where several electric plants fired by natural gas are expected to be built in the next several years.

But fierce opposition has arisen in the United States, particularly in Westchester County, where environmentalists, homeowners, elected officials and others have called the project unsafe and detrimental to the environment. The route in Westchester has been changed twice to appease opponents, but to no avail.

TransCanada Pipelines Ltd. and St. Clair Pipelines, which are building two portions of the pipeline, a 58- and a 60-mile line, from Lake Erie to Dawn, Ontario, sent a letter dated Aug. 14 to Canada's National Energy Board notifying it of the withdrawal, with the option to refile "if and when appropriate."

Kurt Kadatz, a spokesman for TransCanada, based in Calgary, Alberta, said the delays in the United States made it difficult for the Canadian sponsors to attract customers for the gas line there.

Mr. Kadatz declined to say whether the sponsors would refile the application if the United States approves the New York end. But he said the letter left open the option to refile and added: "I reiterate we are still interested in serving that area of the United States. The area is an important one for us."

Karl Brack, a spokesman for Columbia Gas Transmission, which is developing the New York pipeline, said the withdrawal was expected and, while a setback, was not a fatal blow to the project.