Mayor Eric Adams pulled in the city’s fiscal belt another notch this week, ordering most agencies to close thousands of vacant positions as the city looks to close a $2.9 billion budget gap in the next fiscal year — exacerbated by the city’s $1 billion migrant crisis and deals cut with municipal unions.

A Monday letter from Budget Director Jacques Jiha to agency heads obtained by The Post, and first reported by Politico, said the Mayor’s Office of Management and Budget would be reducing full-time positions that were vacant at the end of last month by 50%.

The cuts mean that about 4,700 vacancies would be left unstaffed, but the directive will not affect teachers or uniformed members of the police and fire departments.

The missive also directed agencies to self-fund new initiatives and programs and “submit detailed proposals” outlining the funding by Dec. 5.

The news came after Adams ordered a 3% budget slash in September intended to save $2.5 billion over the next two fiscal years, on top of additional sweeping cuts the incoming mayor made in January.

Jiha cited pending settlements with the city labor unions and New York’s estimated $1 billion bill for sheltering migrants as the reason for the new cuts, in addition to broader economic woes.

“Until the federal and state governments step in, we are shouldering the entire cost of providing mandated assistance to asylum seekers,” he wrote.

“Furthermore, the local and national economies are weakening, financial market performance is down, and we face rising health-care costs, high energy prices, and elevated inflation.”

Fiscal watchdogs cheered the decision.

“It is a prudent move. We have called for them to remove the unnecessary vacancies. The fiscal cliff is real. It’s looming,” said Ana Champeny, vice president for research at the Citizens Budget Commission. 

She noted the city workforce has needed a trim ever since it grew under former Mayor Bill de Blasio. By June 2014 — his first year in office — the city workforce stood at 297,349 — but it ballooned to a high of 326,739 in June 2019. The number gradually decreased during his last two years in office and this past June 2022 recorded a total of 304,095 city employees. 

“If there’s a recession, the uncertainty with collective bargaining and they’re running out of federal aid,  I think it’s important for them to take action now,” said Champeny.

Contract negotiations underway with dozens of unions that represent some 300,000 city workers have been delayed and complicated by legal objections from healthcare providers and retirees who claim the de Blasio administration forced them into a new privatized plan that offers fewer benefits at a higher cost.

“We are in financial trouble,” Adams said when asked about the city’s fiscal situation at a press conference Tuesday morning.

“When I ran for office I used the terms “PEGS” [Program to Eliminate the Gap] … I stated that we have to deliver a better product by using taxpayers’ dollars better,” the Democrat said. 

“I am keeping my campaign promise. I believe that inside our agencies, we have to find efficiencies. We got to do everything we can to find these efficiencies.”

City spending surged about 3% to $104 billion in fiscal year 2023, which began this summer. The city’s nearly $3 billion deficit could swell to $6 billion in fiscal year 2026, according to budget projections.

The increase prompted a dire warning from New York Comptroller Tom DiNapoli, who said the bloated budget “could create spending pressure” in future years, as he called for “balancing” the need to deliver municipal services with filling the city’s coffers.

“Without the hardworking individuals who keep this city running, critical and essential services for our children and most vulnerable residents could be impacted,” DiNapoli warned earlier this month.