The MTA’s massive upcoming deficit could be far worse than officials are predicting — because they’re assuming hundreds of millions of dollars in savings that won’t happen under current worker contracts or if the economy goes south, a new report warned Wednesday.

The analysis from the Citizens Budget Commission asserts that the $538 million in savings the MTA hopes to wring out of its just-approved reorganization plan is “highly optimistic” — in part because the savings can’t be achieved without concessions from the agency’s more than two dozen unions.

“The plan depends on everything going perfectly well, and things will not go perfectly,” CBC President Andrew Rein told the Post. “It’s an incredible challenge.”

In December, MTA Chief Financial Officer Bob Foran warned that the agency faced deficits approaching a billion dollars if it didn’t take immediate steps to find savings.

His July budget update anticipated much smaller deficits — thanks to savings from both the reorganization plan and a separate, ongoing effort to reduce costs.

But many of the supposed savings are vague or limited in details, the CBC charged. Others are heavily dependent on factors out of the agency’s control — such as the state of the economy and the outcomes of negotiations with its 32 different unions.

For example, Foran’s July report said the subways department plans to save $22 million each year starting in 2021 by eliminating 222 jobs — while also admitting the MTA hasn’t actually figured out what jobs to cut.

“They have to hit the ground running, develop the specifics and implement them well,” Rein said.

The reorganization plan, prepared by consultant AlixPartners and approved last month, advised that some cost savings would have to be achieved through labor contract negotiations.

But the plan assumes 2% annual salary increases — and the Transport Workers Union’s most recent contract agreement included raises of 2.5% and a $500 bonus for workers, the CBC noted.

TWU Local 100’s contract expired in May and its next contract will likely set the tone for the negotiations with its 31 other smaller unions.

The CBC report also advised any future wage increases should be offset by concessions elsewhere — such as higher healthcare contributions from workers.

The TWU did not respond to a request for comment.

In a statement, MTA spokesperson Tim Minton acknowledged the MTA’s “financial hurdles,” but insisted that it is up for the challenge.

“We are facing these challenges strategically, with pragmatism, in touch with the realities of both our revenue streams and expenditure needs,” Minton said.