The MTA has given the city a deadline to fund its Subway Action Plan.

The authority will have no choice but to scale down its plan to reverse declining subway service by the end of January, unless the city commits to pay for half of the $836 million, multi-pronged plan, according to Tim Mulligan, the agency’s acting president MTA New York City Transit.

“As we continue, we have to resolve the underlying funding of this program. We will not be able to continue hiring at the pace of the programwe’ve been hiring past probably the end of January,” Mulligan said at the MTA’s Transit Committee meeting on Monday. “So with the budget cycles for the city and the state both coming up, we’re looking for additional appropriations and intentions of commitments of support so we can keep this program going.”

Facing a soaring number of delays and high-profile service outages during peak hours, MTA Chairman Joseph Lhota unveiled the Subway Action Plan this summer. It relies heavily on the hiring of 2,700 new workers to address its core components: track and signal maintenance, car reliability, subway safety and cleanliness and customer communication.

Work toward the plan has already begun, with the MTA tapping into its budget reserves to hire more than 750 workers as of Monday. The agency has now limited its focus on six key corridors through the city, including Lexington Avenue, Eighth Avenue and Sixth Avenue lines in Manhattan as well as routes running through Downtown Brooklyn and the South Bronx.

Lhota has billed the plan as an elixir for the poor subway service in recent years. But since its unveiling, both Lhota and Gov. Andrew Cuomo — Cuomo essentially controls the state-run MTA — have hounded the mayor to provide half of the necessary funding.

The city has pledged $2.5 billion for the MTA’s five-year, $32 billion capital plan, and Mayor Bill de Blasio has stated that he’s unwilling to hand over more money to the MTA until it becomes more responsible with its spending.

The MTA on Monday found support from a reliable source, board member and Transport Workers Union International President John Samuelsen. The labor leader penned a letter to his fellow board members, dated Sunday, Oct. 22, urging them to “take action to rectify this injustice.” He claimed the city has already shirked responsibilities to pitch in money for student MetroCard subsidies and paratransit services.

“NYC cannot continue to have it both ways, simultaneously refusing to contribute toward the Action Plan and being a deadbeat on the reimbursement for services rendered,” Samuelsen wrote in the letter.

De Blasio’s office did not immediately respond to a request for comment.

John Raskin, the executive director of the advocacy group Riders Alliance, shot back, calling the MTA’s attempt to get the city to fund the plan a “transparent attempt to shift blame and accountability for our transit woes onto the Mayor, even though the Governor himself controls the MTA.”

“Governor Cuomo is holding millions of transit riders hostage in his dispute with the Mayor,” Raskin said in a statement, “even after he acknowledged that the subway system is in a state of emergency and promised to fix it.”