The Metro board voted Thursday to essentially cut its size by half — a radical change in the agency’s governance that was approved unanimously, despite several members’ objections to the restructuring.

The move was in response to demands from Virginia lawmakers that the 16-member panel give more power to its eight “principal” voting members and reduce the role of the nonvoting “alternates” as a condition of receiving the state’s share of $500 million a year in new dedicated funding.

The measure passed Thursday bars alternates from participating in board or committee proceedings — a change proponents say is intended to streamline discussions, reduce parochialism, and increase the level of expertise among members of the panel.

But several members, both voting and nonvoting, have raised concerns that the change will hamstring some of the most knowledgeable and engaged members of the transit agency’s governing board.

Even so, all eight principal members voted for the change, saying they feared rejecting the measure would jeopardize the hundreds of millions of dollars promised by Virginia to fund Metro’s infrastructure needs in coming years.

The decision occurred at the same meeting at which board member Jack Evans was elected to a third term as the board’s chairman. It’s the first time in recent memory that a chairman has served more than two terms.

Evans, who is also the D.C. Council member for Ward 2, said after the meeting that he is looking for ways to possibly keep the alternate members involved, or establish a forum where those members can offer their advice or expertise.

For years, lawmakers and other leaders in the region have expressed interest in overhauling the board, which includes four representatives each from the District, Virginia, Maryland and the federal government. Many believed the board’s size was unwieldy and did not serve the best interests of the transit agency. In a report released last year, former U.S. secretary of transportation Ray LaHood recommended the panel be replaced with a five-member “reform board,” but there was little political will to tackle the challenge.

In the end, Virginia lawmakers included a provision in the state’s dedicated funding law that bars alternates from participating in board meetings unless they are sitting in place of an absent board member. Previously, alternate members participated in discussions and voted on resolutions at the committee level, though they were barred from voting on matters that reached the full board.

The alternates include members widely considered to be assets to the board, including Robert Lauby, chief safety officer at the Federal Railroad Administration.

The funding law takes effect July 1.

In an awkward opening to Thursday’s meeting, alternate Malcolm Augustine took the only opportunity he had to publicly express his outrage at the restructuring: He signed up to speak during the public comment period. When his name was called, he stood up from his seat in the audience, approached the podium and used the two minutes that he was allotted to speak.

“I strenuously object to the changes in the bylaws that you are considering, which will basically circumvent the compact that governs this body,” said ­Augustine, who represents Prince George’s County. “Virginia is holding all of us hostage, and it will disenfranchise Prince George’s County.”

Augustine said he was concerned that removal of the alternate board members, who are assigned to represent specific jurisdictions, would leave those communities without a voice on Metro matters that might disproportionately impact them.

“I think you’re going down a wrong path,” he said.

Board member Michael Goldman tried to rally support for an amendment that would have limited the extent of the changes, but failed.

“We should do this in a more comprehensive, more structured, more timely manner,” Goldman said.

Board member Christian Dorsey also said he was disappointed by the changes. Dorsey had been an alternate until last month and had been vocal in criticizing the decision to sideline board members who he said had spent years working on behalf of the agency.

The Northern Virginia Transportation Commission recently voted to make Dorsey a principal board member, replacing Catherine Hudgins, the lone woman among the principal members of the board. Hudgins was made an alternate.

“It’s not going to serve this board well, it’s not going to serve this agency well, and it’s not going to serve the public well,” Dorsey said. “These are tangible, real contributions that somehow we’re going to have to backfill.”

But David Horner, a federal representative on the board, said the change will ultimately result in more experienced and knowledgeable board members.

“Over the long term, the jurisdictions will compensate for the supposed loss of access to expertise by putting forward, as members of the board, individuals who possess levels of expertise and experience of complex organizations that few, if any, members of the board today possess,” he said. “At the end of the day, that is a better model for governance of a complex transit property.”