The California Public Utilities Commission warned rideshare companies early this month that they are not allowed to operate at the state’s major airports without airport permission.

The commission says rideshare companies need permits from airport authorities. A San Diego International Airport spokeswoman said June 24 that the airport is in initial talks with the rideshare providers — also called transportation network companies or TNCs — about permits, but that the airport has issued no such permits.

In September, the Public Utilities Commission gave TNCs permission to operate under a state permit if they meet certain conditions. Letters sent to TNC officials, dated June 10 and posted on the commission’s website, threaten to revoke the state permits if the TNCs don’t comply with the commission’s rules.

The state called for compliance by June 24 and has threatened random audits of the companies.

Established taxi companies, who compete with TNCs, say rideshare drivers try to operate discreetly at airports by removing distinctive identifying markings from their cars.

TNC drivers link up with their passengers through a software application.