Uber has been ordered to remove its 'illegal' self-driving cars from the roads of San Francisco, less than a week after the company began testing them.
According to the Department of Motor Vehicles (DMV), Uber requires a test permit to carry out its trials.
The attorney General of California has ordered that Uber stop test-driving the vehicles immediately, or face legal action, which could potentially mean a court order.
However, Uber has argued that it does not need a permit as its cars have safety drivers at the wheel, and plans to ignore the demand to cease tests.
In a letter to the Taxi app firm this week, Brian Soublet from the California Department of Motor Vehicles wrote: "It is illegal for the company to operate its self-driving vehicles on public roads until it receives an autonomous vehicle testing permit."
In response, Uber's vice-president of Advanced Technologies Anthony Levandowski held a conference call with media on Friday. During this call, he said that while his company has "respect for officials", the regulations were irrelevant to its cars, adding that in his view, the permit only applies to vehicles "without the active physical control or monitoring of a human operator".
Levandowski likened the Uber technology to Tesla's Autopilot feature, which doesn't require a permit but can take control of the vehicle to automatically keep up with traffic and change lanes.
To obtain the permit in question, a company must apply and pay only $150 (£120). This permit will allow 10 vehicles to be tested on the roads, and further cars can be added at a rate of $50 (£40) for each additional 10 vehicles.
It requires companies to cover insurance costs and provide public reports on any collisions or instances when the human driver must take control.
Google is amongst other companies to have obtained the permit to test in California.