The MTA’s $51.5 billion capital plan inched closer to reality on Wednesday thanks to endorsements from both Mayor Bill de Blasio and the transit agency’s board.

Board members unanimously approved the five-year funding strategy — the MTA’s biggest ever — despite concerns that they weren’t given sufficient time to review its contents.

“To be where we are at this moment is in fact monumental. It is in fact historic and worthy of congratulations,” board rep Veronica Vanterpool, a de Blasio appointee, said before voting in favor of the plan — which includes over $41 billion in funding for the city’s buses and subways.

At the same time, Vanterpool called communication with board members and the public regarding the plan, which was not released until last Thursday, “wholly inadequate.”

“This plan was conceived in the dark,” added Orange County rep Susan Metzger, who also voted yes despite saying the timing was “way too late” for the plan’s contents to be properly evaluated.

To fund the $51.5 billion plan, the MTA is counting on billions from congestion pricing, tax revenues, the federal government and borrowed debt — plus another $3 billion each from the state and the city’s respective coffers.

In a letter to MTA Chairman Pat Foye dated Wednesday morning, de Blasio confirmed the city would cough up cash for the plan — pending a “full review” of its contents that “yields guarantees” the plan is as cost-efficient and feasible.

He stopped short of committing to the full $3 billion, however, other than to note the city contributed $2.6 billion to the last plan.

“New York City stands ready to contribute to an MTA capital plan, as it has done in the past,” the mayor said.

Foye welcomed de Blasio’s comments, which he called an “important development.”

With the board’s support sealed, the plan now moves to the four-member Capital Program Review Board, which includes representation for the mayor, the governor and each house of the state legislature.

That’s likely to push final approval into 2020, when the city and state complete their respective budget processes.