A standoff between Uber and the State of California became more serious on Friday when Uber rejected a demand to get permits for its self-driving car program and the state fired back with a threat of legal action.

The spat began earlier this week when Uber launched a fleet of self-driving cars in San Francisco. The company had not obtained autonomous driving permits for the cars, and California’s Department of Motor Vehicles asked the company to stop the service until they were issued.

On Friday, Uber said no.

“We respectfully disagree with the California Department of Motor Vehicles legal interpretation of today’s autonomous regulations, in particular that Uber needs a testing permit to operate its self-driving cars in San Francisco,” Anthony Levandowski, vice president of Uber’s advanced technologies group, told reporters.

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