ALBANY – The state comptroller urged the Bloomberg administration yesterday to temper its enthusiasm over New York City’s current budget surplus, as projected gaps in future budgets and a short list of giant potential expenses threaten major financial problems for the city in the years ahead.

In two separate reports timed to coincide with a meeting this week of the city’s Financial Control Board, the comptroller, Alan Hevesi, credited Mayor Bloomberg and other city officials for an estimated $3.5 billion surplus in the fiscal year that ended June 30. The largest ever surplus resulted from revenue spikes in taxes on personal income, business, and real estate transactions.

Mr. Hevesi also warned, however, that the city’s finances are jeopardized by multibillion-dollar gaps in future years due to increasing pension costs and a rapidly growing debt burden. The comptroller said city officials, who are balancing next year’s budget with this year’s surplus, cannot expect a favorable economic climate each year to cover anticipated losses.

“What the report is saying is that the city has done a good job managing its finances,” a spokesman for Mr. Hevesi, John Chartier, said. “We’re in good shape for 2006, but we need to be aware that there are some big decisions that have to be made in the ‘out years’ and that the city has its work cut out for it. Now is the time to start looking at those out years.”

Mr. Hevesi, as comptroller, does not offer specific advice on the steps municipalities should take to address financial concerns. Independent analysts said yesterday that the city administration will have to make politically unpopular decisions if it expects to dull the impact of a future economic downturn. Mr. Bloomberg, who is up for reelection this fall, is unlikely to advertise his position on such measures now.

“There certainly is a scenario in which the very, very robust revenue growth the city has seen in the last year continues, and that definitely improves the picture,” the president of the independent watchdog group Citizens Budget Commission, Diana Fortuna, said. “But that doesn’t eliminate the problem. You can’t rely on double-digit growth in real estate-related taxes forever.”

The Bloomberg administration, which reported numbers that differed slightly from the comptroller’s, blamed its future gaps on Medicaid and pension costs mandated by the state. Roughly 13% of New York City’s $50.2 billion budget went to Medicaid, the health care program for the poor and disabled.

“The principal force in the out-year gaps is the mandated spending from Albany,” a spokesman for Mr. Bloomberg, Jordan Barowitz, said. “Agency spending is flat and has been for three years. What’s driving up the city’s budget are pension costs and Medicaid.”

A budget analyst with the Manhattan Institute’s Empire Center for New York Policy, Edmund McMahon Jr., said he agreed, but only in part, with the administration’s analysis of its financial straits.

“If you’re spending more than you can afford, then one way or another, you’ve got to spend less,” Mr. McMahon said. “They need to have a leaner work force, to cut back on programs that are not core services, and to be more politically active in ending state mandates, which is something they have never really pressed for. The city is out on a limb, and all the fiscal monitors are pointing to the same risks. Bloomberg lucked out in the last year, but he’s going to need to be very lucky to avoid significant reductions next year.”

The comptroller’s report identified a single additional revenue source and an accounting change that could help narrow gaps in future years: a projected $770 million surplus in fiscal 2006, and the delayed reporting of investment returns from the city’s pension fund – a move that Mrs. Fortuna referred to as “a gimmick.” The report also warned that current budget gap projections could grow even larger if a decade-old court case mandating billions for city schools is resolved.

The city’s current projections take into consideration reductions in Medicaid spending that passed the Legislature earlier this year, including a cap on county contributions and a preferred drug list. Nondiscretionary items such as Medicaid, pensions and benefits, and debt service ate up 37% of the city’s revenues in 2003 and are expected to consume more than half by 2009.

Looking forward, Mrs. Fortuna said some difficult decisions will have to be made to stem the spending pattern soon.

“They took a break from the heavy lifting the budget really needed this year,” she said. “But I think a year from now we are going to be a talking about a lot more tough actions.”