Mayor Gavin Newsom has accused some of the city's cable car conductors of pocketing fares, stirring the ire of the union representing them.

"I want a formal apology from the mayor," said Antonio Gonzalez, the cable car division chief for the Transport Workers Union, Local 250A, which represents the city's 174 full-time cable car operators.

"For the mayor to come out and make this statement, it's uncalled for," Gonzalez said Thursday.

The mayor, with little to back him up, made the allegations Wednesday during a budget briefing at his office. He said he believes some cable car conductors are skimming fares from cash-paying riders because on three occasions he rode the cars and handed over his $5 cash fare but never received the required receipt.

Newsom's spokesman Peter Ragone said Thursday the incidents occurred during the past year. He said the mayor occasionally rides the cable cars dressed down, wearing a cap and sunglasses. One time in particular, he said, he boarded the car with a group of tourists.

While Newsom stressed that most cable car operators are honest, he said he observed a conductor collecting five or six fares on a packed cable car "and didn't give one receipt for any of them."

"I am convinced, based on my own personal experience, (the money) is not going to Muni, but it's going in the pockets of some of our well-meaning operators," Newsom said during the meeting.

Cable car operators collect $5 fares from passengers after they've boarded the train. They give each person a receipt and at the end of the day, the amount of cash they turn in must match their receipts. Some riders do buy tickets at kiosks, but there are only three booths in the city and only two of them operate on weekends, often the busiest time for cable car-loving tourists. Theoretically, drivers could pocket the fares without issuing a receipt because there's no other way to account for the money.

Union leaders were incredulous at the mayor's allegations.

"If he witnessed incidents, there are channels for reporting those incidents," said local union President Irwin Lum. "We're shocked and outraged."

Ragone said the mayor did report the problems to the Municipal Transportation Agency.

"The mayor feels that this is not a widespread problem. However, it's apparent it has occurred, and the mayor, the MTA and the union have been working on alternatives to the cash-based system we currently have."

Officials at the Municipal Transportation Agency contacted the union about two weeks ago to discuss revamping the fee collection system for Muni, which operates the city's entire public transit system, including buses, trains, trolleys and cable cars, Lum said. But it wasn't prompted by allegations of theft. It was intended to shorten wait times and make collecting fares more efficient, he said.

A Muni spokeswoman did not immediately return a call seeking comment.

People board cable cars nearly 8 million times a year, according to Muni's Web site. In the fiscal year ended June 30, 2005, cable car fares amounted to $16.2 million or nearly 14 percent of Muni's total revenue. Of that $16.2 million, conductors collected about 46 percent or about $7.4 million aboard the trains, according to Muni's audited financial statements.