The staggering economic fallout from the COVID-19 pandemic is expected to create a budget deficit in San Francisco of from $1.1 billion to $1.7 billion over the next two fiscal years, city officials said Tuesday.

The grim projections, released by Mayor London Breed and City Controller Ben Rosenfield, accompanied an announcement that San Francisco’s budget-setting process would be delayed for two months to buy the city’s financial experts time to readjust their spending plans in light of stark revenue losses.

In December, the projected budget shortfall over the next two fiscal years was pegged at around $420 million. That gap between the city’s spending plans and available revenue has roughly quadrupled. Last year’s budget, the largest in the city’s history, was $12.3 billion.

“The coronavirus pandemic is an immediate threat to our public health, and we’re doing everything we can to slow its spread and save lives, but we know that it is also having a major impact on our economy and our city’s revenue,” Breed said in a statement.

Although the city is seeking more money and other resources from the state and federal governments to help support people who have lost their jobs and had to close their businesses, Breed said San Francisco would need to “make the hard choices to get through this and to get San Francisco back on the road to recovery.”

The city has already sustained substantial losses brought on by the threat of the coronavirus and its attendant impact on the economy. The estimated losses reflect evaporated revenue the city otherwise would have expected to receive.

Over the next three months, city officials expect a shortfall of from $167 million to $288 million, driven primarily by losses in hotel and real estate-transfer taxes. The 2020-21 fiscal year is shaping up to be worse, according to the projections, with $330 million to $581 million in revenue drained away. Losses in the 2021-22 fiscal year are estimated at between $214 million and $382 million.

The projected deficit for the next two fiscal years also includes the city’s previously estimated shortfall of $420 million.

Breed sent a letter to department heads Tuesday laying out her immediate directives for curbing city spending, as her budget office spends the next month working to close the budget gap.

While the city continues expending time and resources toward its coronavirus response, departments were instructed to freeze their hiring plans and add only new employees working on “essential services” related to the city’s coronavirus response or those performing critical tasks, such as street cleaning.

Any nonessential capital projects — like maintenance to buildings, nonemergency tree pruning or street-repaving — funded in the current fiscal year also will be paused, as will any programs that have not yet started.

The Board of Supervisors also will shorten its annual summer recess. Supervisors will convene throughout the month of August to meet the new budget deadlines, President Norman Yee said in a statement. City officials said San Francisco’s budget will be completed and signed by Breed by Oct. 1, two months later than usual.

“The months and years ahead will be challenging, but we have risen out of crisis before, and we will need to make critical decisions that will lead us back to fiscal health,” Yee said.

Dominic Fracassa is a San Francisco Chronicle staff writer. Email: dfracassa@sfchronicle.com Twitter: @dominicfracassa