For the first time, San Francisco is going to require the 37,000 Lyft and Uber drivers who work in the city seven or more days a year to obtain a business license.

City Treasurer Jose Cisneros wouldn’t fully explain why he is now requiring the license, which will cost drivers $91 annually, when the companies started operations years ago. But one reason, he said, is that the city launched its online business registration system in March — before, registrants had to go to City Hall to apply in person.

The move ups the political tension between the city and Uber and Lyft. When faced with class-action lawsuits from drivers seeking status as employees, the companies have vigorously maintained that the drivers are independent contractors. Cisneros is in essence turning that argument back on them and saying: If that’s the case, the drivers have to register as independent contractors for a business license.

Another reason the treasurer is taking action now may be that he finally has the names of the drivers. Uber and Lyft have long refused to provide drivers’ names and addresses to the city. Cisneros would not say how he obtained them.

The license will cost drivers $91 a year if they earn $100,000 or less in gross receipts. If they have been driving for multiple years, they will have to pay a registration fee for the years in which they didn’t register.

Cisneros’ letter, which will be sent in three batches — on Friday, Monday and Tuesday — states that the recipient has been identified as a driver for a transportation network company and therefore must obtain a business registration certificate within 30 days.

“Failure to respond to this letter may result in penalties and payment obligations,” the letter states.

“We have a very broad and comprehensive business registration requirement,” Ciseneros said. “This has been a law that has been around for many years. It’s very clearly spelled out on our website — the law here in San Francisco requires you to register your business with the city. If they missed that requirement, they are still obligated to do that.”

In response to the news, Uber struck a conciliatory tone and indicated it would not challenge the city.

“Uber partners with entrepreneurial drivers and as independent contractors, they are responsible for following appropriate local requirements,” Uber spokeswoman Laura Zapata wrote in an email.

Lyft spokeswoman Chelsea Wilson said the company is opposed to the plan.

“We have serious concerns with the city’s plan to collect and display Lyft drivers’ personal information in a publicly available database,” she said in an email. “People in San Francisco, who are choosing to drive with Lyft to help make ends meet, shouldn’t have to compromise their privacy in order to share a ride.”

Cisneros has aggressively gone after companies that profit from the gig economy, sometimes clashing with Mayor Ed Lee in the process.

In 2012, over Lee’s objections, Cisneros ruled that Airbnb owed back taxes, which city officials estimated at $25 million. The mayor wanted Cisneros to hold off on the decision until a broader tax overhaul could be rolled out. Airbnb acquiesced last year and paid the taxes.

Lee was not involved in Cisneros’ decision to require the Uber and Lyft drivers to register with the city.

Christine Falvey, Lee’s spokeswoman, neither endorsed nor opposed the move. “The mayor defers questions about the definition of independent contractor and interpretation of requirements under city law to the treasurer,” Falvey said.

Cisneros said he doesn’t expect that all 37,018 drivers who receive the letter are still drivers. But if they are and they register, it will generate $3.37 million a year for the city. An unknown number of drivers already have the license.

It is also unclear to what extent Cisneros will be able to enforce the business registration requirement. He said San Francisco law requires firms to display a registration certificate in their place of business — including their car — and drivers could be cited for failing to do so.

Both Lyft and Uber have faced class-action lawsuits over the past year over whether its drivers are employees or independent contractors. Plaintiffs say they should be classified as employees and entitled to reimbursement for expenses as well as the rights and benefits of employees.

Lyft attempted to settle such a lawsuit in January, agreeing to pay $12.25 million to drivers, although not classifying them as employees. A San Francisco federal judge rejected that proposed settlement this month, saying the $12.25 million shortchanged the drivers. A similar lawsuit against Uber is still in the courts.

“It’s incredibly important to Lyft and Uber’s business model that the drivers be independent contractors,” said UC Hastings law Professor Reuel Schiller. “They are not really interested in compromise. This sounds to me like the city of San Francisco is attempting to play hardball in return.”

Emily Green is a San Francisco Chronicle staff writer. Email: egreen@sfchronicle.com

Twitter: @emilytgreen