California regulators say Uber, Lyft and Sidecar's car-pool services are illegal

These companies are no stranger to regulatory roadblocks, including an outright ban on Uber's UberPop service in Germany, as they launch into new markets. But Uber responded with aplomb to the California commission's letter.

"We thought we had seen it all, and then the CPUC decided they would try to shut down app-based car-pooling," said Uber spokesman Lane Kasselman. "The only conclusion we can come to is that the CPUC doesn't like technology, environmental progress, or anything that might make California a better place to live."

That might be putting it a bit strongly. Late last year, California authorities approved new regulations allowing companies such as Uber and Lyft to operate legally. However, those new regulations apparently don't apply to car-pooling.

A spokesman for the CPUC did not say how the agency would respond if it was not able to reach an agreement with the companies.

However, the agency's action could throw a wrench into the companies' plans to grow the use of their services in new areas.