Chariot, the consumer-shuttle service that was forced by regulators to suspend operations in San Francisco on Thursday, had an unknown number of improperly licensed drivers during three separate safety inspections, according to the California Highway Patrol, which conducted the reviews.

That issue, rather than a mechanical failure, triggered the California Public Utilities Commission to suspend Chariot’s operating permit.

The company’s 15-person vans require “drivers to have a specific license: either a class B or a class C with passenger endorsement,” said Sgt. Robert Nacke of the California Highway Patrol. Most regular drivers have a class C license.

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The CHP conducted three consecutive inspections of a random sample of Chariot vehicles and drivers several months apart starting last October, Nacke said. Each time, inspectors found at least one “out of class” driver working for the service.

“Our inspectors brought it to their attention and said you have to make sure your drivers have the proper training and skills,” Nacke said.

After a third successive failed inspection, the CHP was obligated to submit a formal recommendation to the CPUC, he said. That triggered the shutdown order.

“The CPUC has a responsibility to suspend a carrier’s operating permit for failure to maintain a vehicle in safe operating mode, other violations related to transportation safety, and/or failure to comply with the DMV’s employee Pull Notice Program,” said spokesman Christopher Chow in an email.

Chariot, which is owned by Ford Motor Co., said in a statement that it was “in full compliance with all regulations” and disagrees with the CPUC order but will abide by it while trying to resolve the matter as quickly as possible.

Carolyn Said is a San Francisco Chronicle staff writer. Email: csaid@sfchronicle.com

Twitter: @csaid