The district attorneys of San Francisco and Los Angeles have sent a joint letter to Uber, Lyft, and Sidecar, threatening the popular quasi-taxi companies with a lawsuit if they do not make some modifications to their services.

Specifically, the letter, which was sent on Wednesday, says that the firms are in violation of state law that prohibits charging individual fares for drivers who pick up separate passengers traveling in the same direction at a lower price (essentially as a quasi-bus service), and for implying that their background checks of drivers extends beyond seven years in the past.

"Each of these measures can be implemented quickly, easily, and without impacting Sidecar's ability to operate," the district attorneys wrote, according to a copy of the letter Sidecar sent to Ars.

Jenna Richard, a Sidecar spokeswoman, told Ars in a statement that the company "strongly disagreed" with the position of the district attorneys.

"Shared Rides are great for California because they are safe and affordable, cut down on traffic congestion, and reduce pollution," she wrote by e-mail. "The District Attorneys are trying to enforce laws written for limousines, in an era before smartphones. Sidecar will continue to operate and expand Shared Rides."

She added further in a separate message that the company was setting up a meeting with the two DA offices "to understand their concerns." Richard also noted that the company was "committed to rider and driver safety" and has maintained a 10-year background check, but in order to comply with the CPUC's rules, reduced that to a seven-year check instead.

"In the meantime, we are continuing operations of Shared Rides and maintaining the current wording of the website," she said.

Uber and Lyft did not respond to Ars' request for comment.

"Overzealous regulators!"

None of the three companies have made public statements on their corporate Twitter accounts or blogs, but Sidecar’s CEO Sunil Paul also posted an online petition encouraging customers to "ask the CPUC and the DA offices to end their campaign to stop Sidecar Shared Rides, UberPool, and Lyft Line in California. Overzealous regulators, acting under pressure from big taxi companies, are trying to shut us down."

Over a year ago, the California Public Utilities Commission, the agency responsible for regulating taxis and limousines in the Golden State, legitimized these firms as "Transportation Networked Companies" (TNCs).

While the firms have met with some resistance from regulators in other cities nationwide (and around the globe), California seems to have largely embraced them.

Since the introduction of ridesharing companies in 2012, California’s transportation regulator has consistently scrutinized the practice. By late 2012, CPUC slapped three firms with a cease-and-desist. Next came fines, which didn’t do much to stop the companies’ operations. By December 2012, the CPUC seemed amenable to changing the rules as a way to legitimize these businesses.

"We value innovation and new modes of providing service to the public," San Francisco District Attorney George Gascon said in a statement on Friday. "However, we need to make sure the safety and well-being of consumers are adequately protected in the process."