A coalition of Northern Virginia business groups on Thursday joined the growing ranks of regional leaders calling for cost reductions to the multibillion-dollar project to extend Metrorail to Dulles International Airport and Loudoun County.

The call for belt-tightening cranks up the pressure on the Metropolitan Washington Airports Authority, which is managing the project, at a time when the authority is engaged in negotiations to try to trim the cost of the second phase of the project. Construction is underway on the first phase through Tysons Corner to Wiehle Avenue in Reston.

Standing across the street from the construction site at Wiehle Avenue, business leaders said the future Reston station could not be the end of the line.

“We’re concerned, not just that it’s expensive, but that the project could collapse under its own weight because it’s just not feasible,” said Tony Howard, president of the Loudoun Chamber of Commerce. “They need to dial it back.”

The business groups represent employers and employees in Fairfax and Loudoun counties who are major underwriters of the project, estimated to cost $3.5 billion. Employers are paying for the project through real estate taxes and business licensing fees, while Dulles Toll Road drivers would be on the hook for its rising rates.

In a letter to the airports authority board, the group recommends shrinking the size of a planned rail yard at Dulles; entering public-private partnerships to build parking structures; and swapping a planned underground station at Dulles for a less expensive above-ground location. In addition, the group said it objects to plans that would require the lead contractor on the project to hire trade union workers, a provision the coalition considers anti-competitive.

Charles D. Snelling, chairman of the board, rejected the assertion that a labor agreement would be anti-competitive, saying it had saved money and time in the first phase of the project. But he said the board is considering a long list of potential savings in its discussions that continue Friday at the U.S. Department of Transportation.

“We know money has to be saved,” he said. “And the burdens need to be shared equally.”