The city, already facing a $1.5 billion unfunded pension liability, could be forced to push back its 2025 deadline for paying off the massive debt due to millions the hefty recent patrolmen’s pay hike will add to Boston’s retirement costs, a fiscal watchdog warns.

Matt Cahill of the Boston Finance Commission said the city’s self-imposed deadline of a fully funded retirement system in 11 years would inevitably be delayed unless the city finds more money to put into the fund — by hiking taxes or upping the rate of return on its investments.

“It would have to (be postponed), unless you revamped your financial outlook,” Cahill said. He suggested Meredith Weenick, the city’s chief financial officer, “will have to go back to the drawing board and come up with another way of paying (the increases) or extend the period of time.”

Sam Tyler of the Boston Municipal Research Bureau warned it would be a “huge mistake” for the city to push back the target date since another economic downturn could wreak havoc on the system.

“The city has to stay on course,” Tyler said, praising Boston for its annual 9.25 percent increase in unfunded pension payments, the most aggressive outlay among the 105 municipal retirement systems in the state. “It will be up to the next mayor to determine whether the increases can be accommodated within the operating budget or if the city will have to extend the full funding (deadline) by another year or two.”

Under a new state pension reform law, municipalities have until 2040 to fully fund their retirement systems. There is no law for fully funding retiree health insurance liabilities. Weenick told the Herald, “My anxiety is around total unfunded liabilities because I have to pay them both down. It’s worrisome. It’s not worrisome like it will drive the city to ruin, but we need to remain committed to paying down our pension liabilities so we can then more aggressively pay down our unfunded retiree health benefits.”

Weenick said she will not know the full financial toll the 25.4 percent police pay hike will have on the city’s pension debt until a private firm conducts its regular, two-year evaluation next spring. Weenick said the city has 10,000 retirees, including 1,864 police officers. The city’s annual unfunded pension payments have risen from $126 million in fiscal year 2012 to a projected $168 million next year.

Overall, the new six-year contract for 1,447 rank-and file cops will cost the city $87 million.

“The whole $87 million is pensionable,” Weenick said, but she added it is “premature” to say whether the police raise could push back the 2025 deadline. “Will it have an impact on the pension liabilities? Absolutely. The clear objective of the arbitrator was to increase pensionable earnings for police officers.”

The pay raise will boost the average base salary of a Boston cop by nearly $20,000 in four years – from $76,372 in fiscal year 2012 to a projected $95,770 in 2016 — driving up their future pension costs. The average patrolmen currently takes home nearly $110,000 with overtime and detail pay, both of which are not counted toward pensions.

Mayor-elect Martin J. Walsh is already on track to inherit $4.5 billion in total unfunded liabilities — including $3 billion in projected retiree health insurance costs. Walsh spokeswoman Kate Norton said his transition team is examining the police contract’s “budget implications.”