By Mara Gay 

New York City is more than $1 billion short of the reserve funds it needs to weather a financial crisis, according to city Comptroller Scott Stringer.

A report prepared by Mr. Stringer's office found that the city has about $8.5 billion in reserves it can use in a bad economy, or about 10% of the city's $80.65 billion adjusted operating budget in fiscal 2016, which began July 1.

"New York City is making progress toward increasing our budgetary cushion, but we've got a good way to go before we're back in a truly secure position," Mr. Stringer said.

The reserves grew by $3.8 billion in the past year and a half as Mayor Bill de Blasio added funds in an improved economy.

But Mr. Stringer, the city's chief financial officer, said the reserves should be between 12% and 18% of the city's annual adjusted operating budget.

Officials in Mr. Stringer's office said they arrived at that range by reviewing the city's performance during the downturn after the Sept. 11 attacks in 2001 and the latest recession.

A spokeswoman for Mr. de Blasio noted the increase in the reserve funds since the mayor took office. "Fiscal responsibility is a cornerstone of the de Blasio administration," said Amy Spitalnick, the spokeswoman.

Mr. Stringer, who like Mr. de Blasio is a Democrat, has emerged as a sometime-critic of Mr. de Blasio.

Last month, he came out in opposition to bill backed by Mr. de Blasio aimed at car-service giant Uber Technologies Inc. that would have temporarily capped the number of livery cars in the city. The proposal was tabled after the city reached an agreement with Uber.

Mr. Stringer released the report ahead of a Financial Control Board meeting scheduled for Tuesday. The state board has the power to take over the city's finances if it fails to balance the budget.

During the early stages of the recession in 2008, the city had amassed about $10.9 billion in reserves, or about 17% of its adjusted operating budget, funds former Mayor Michael Bloomberg used to help balance the budget. By mid-2013, the city had depleted $6.2 billion of those funds, according to they city comptroller's office.

Carol Kellermann, president of the Citizens Budget Commission, a nonpartisan civic organization, said Mr. Stringer's call to boost the reserves further is "an excellent idea."

"We're not prepared for a recession," Ms. Kellermann said.

Write to Mara Gay at mara.gay@wsj.com