A possible fare hike to be announced as early as July is angering straphangers and transit advocates, who say riders should not have to carry the burden of the Metropolitan Transportation Authority’s ballooning operating deficits.

A new report by the city’s Independent Budget Office forecasts that in a best-case scenario, subway and bus fares would have to rise to $2.40 from $2 a ride by 2010 for the transit agency to close the $1.8 billion funding gap in its operating budget projected for 2010.

The report, commissioned by the Straphangers Campaign, says fares would have to jump to almost $3 — an increase that would mark the largest ever fare hike in dollars — if state subsidies and tolls are not also increased by 20%. Monthly Metro-Cards could rise to $112 from $76.

“It’ll take real work just to keep it down to $2.40,” the chief attorney of the Straphangers Campaign, Gene Russianoff, said. A fare hike is something the agency is considering, the CEO and executive director of the MTA, Elliot Sander, said yesterday at a breakfast sponsored by Crain’s. He has said in the past that a fare hike would be an option of last resort. The MTA would announce any plans for a future fare hike next month, when it rolls out its financial plan for the next four years, he said.

A booming real estate market that has yielded larger-than-expected tax revenues for the transit agency has allowed the MTA to stave off a fare hike for years, Mr. Russianoff said, masking the structural deficit in its operating budget.

While the base fare has not been raised since 2003, when it jumped to $2 from $1.50, straphangers yesterday said they were not keen on opening their purses for the MTA.

“Don’t we already pay the MTA with our taxes?” a student, Angelica Cortes, said while waiting for the no. 1 train.

“Sometimes the subway doesn’t work, it’s overcrowded, and the stations are filthy,” a parent coordinator, Kathy Sussell, said. “I think it should be cheaper and better service.”