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In September, the California Public Utilities Commission (CPUC) voted unanimously to adopt a set of regulations dealing with ridesharing companies. It was a major victory for companies like Uber and Lyft, because the rules brought much-needed legitimacy to their operations.

But now the CPUC is threatening to revoke ridesharing companies' authority to operate in California if they continue to thwart airport rules.

In a strongly worded letter sent to ridesharing companies, CPUC President Michael R. Peevey wrote that the CPUC spoke to officials at five California airports – including Los Angeles International Airport – who reported ridesharing drivers who were not properly permitted, didn't have proof of insurance, and even lacked driver's licenses.

"I would like to express my personal disappointment and concern for this behavior," scolded Peevey. "California is the first state that creates rules for this industry to promote consumer choice, we will not, however, accept consumer choice at the expense of consumer safety."

Airports are the last bastion of protected territory for taxicabs, who's drivers typically earn a large percentage of their earnings on the one assigned day a week they're allowed to be at LAX.

The battle with ridesharing companies has been more pronounced at San Francisco International Airport –where there have been reports of drivers being subject to citizens arrests for trespassing – than at LAX.

Lyft didn't respond to a request to comment. A spokeswoman for Uber said Peevey's letter only deals with the lower-cost UberX, where drivers typically use their own cars. UberX already stopped picking passengers up at LAX in January after authorities stepped up their enforcement at the airport.

"Unfortunately, authorities have taken an aggressive stance against UberX and have begun issuing citations to some drivers who pick up passengers at the airport," former Uber spokesman Andrew Noyes said in a January blog post. "That’s why we’re temporarily halting UberX pick ups at LAX effective immediately."

Belinda L. Nettles, a spokeswoman for the Los Angeles Airport Police, said Uber, Sidecar and Lyft as well as other ridesharing companies are still allowed to drop off passengers at LAX.

"The question of picking up passengers by [ridesharing companies] is still under review by the PUC," Nettles said in a statement. "However, the PUC does require that the TNC’s follow all of the rules and regulations [at all LA airports], including acquiring the proper licensing, permits, and insurance that LAX and other airports require. Airport Police Officers currently enforce the airport rules and regulations, the Los Angeles Municipal Codes, penal code and vehicle code and violations of such, as they pertain to TNC’s like Uber, Sidecar, and Lyft."

Nettles couldn't say how many citations ridesharing drivers have received at LAX, information Uber also declined to provide. The company did, however, release a statement expressing its displeasure with the CPUC's letter.

"For months, Uber has met with airports across California," Uber said in a statement. "During this time, we have spent countless hours with airport managers and their staff to help develop requirements that are aligned with CPUC regulations and embrace the spirit of ridesharing. The tone of the majority of these conversations has been mutually respectful and productive. It’s unfortunate the CPUC is not allowing the airport process they designed to proceed by allowing ridesharing companies to continue working with California airport authorities on their permitting process.”