Uber has no plan of ending a new test of its self-driving vehicles in San Francisco even though California regulators see the service was illegal, the BBC reported Saturday.
The company started testing the vehicles this week but the California regulator warned that the firm must have a test permit.
California's attorney general has threatened to act further if Uber does not cease the driving immediately.
The specific action is not clear but it is likely a court order compelling Uber to carry out the demand, according to the BBC.
California law defines autonomous vehicles as those that drive "without the active physical control or monitoring of a natural person," according the New York Times.
Uber said its modified, self-driving car requires human oversight, and therefore does not fit California's definition of an autonomous vehicle, the New York Times reported.
Other companies testing autonomous technology in California, such as Google, have applied for and obtained the permit which costs 150 U.S. dollars for 10 vehicles, the BBC reported.
For seven years Uber's stance on complying with regulations has been consistent: ask forgiveness, not permission, the New York Times said.