Tesla has "pushed the envelope" on safety, former business partner Mobileye said. Its zeal to extend its Autopilot system had led Mobileye to stop working with Tesla, said the chairman of the computer vision company.
Autopilot has been under scrutiny since a fatal crash in May caused by the technology missing a lorry. Responding to Mobileye, Tesla said Autopilot was built to help drivers, not turn cars into autonomous vehicles.
Mobileye executive Amnon Shashua told Reuters that the design of Autopilot threatened to push it beyond the point that it was safe to use.
"It is not designed to cover all possible crash situations in a safe manner," he said. Mr Shashua is also technology chief at Mobileye, which develops driver assistance systems.
"It is a driver assistance system and not a driverless system," he said.
Autopilot was introduced late last year as a way to help Tesla owners stay inside lanes and keep pace with traffic on motorways and other major roads.
Tesla and Mobileye stopped working together in July this year following the fatal crash.
In response to Mr Shashua's comments, a Tesla spokeswoman said it had always sought to describe Autopilot as an aid for drivers rather than something that could replace them.
She said the company had "continuously educated" customers on the safe use of Autopilot, reminding them to keep their hands on a car's steering wheel at all times and to remain alert.
"Drivers must be prepared to take control at all times," added the spokeswoman.
Mobileye's criticism comes only days after Tesla Elon Musk announced changes to the way Autopilot worked, to make it safer to use.
One change involved repeatedly telling drivers to keep their hands on the wheel.
If these warnings were ignored, the assistance system would be turned off until his car was parked.
The changes could probably have prevented the fatal crash in May, said Mr Musk.
The US National Highway Traffic Safety Administration is still investigating the May fatality.