The African-Americans said that Tesla supervisors and workers used racial epithets and drew racist graffiti on cardboard boxes.

This is the second case by black employees charging Tesla failed to tackle racial antagonism at its factory. It also faces a hearing before the National Labour Relations Board over claims it illegally tried to silence workers promoting a union.

The complaints come as Tesla heads into a crucial ramp-up of Model 3 production, its lower cost electric vehicle.

A Tesla spokesman denied the suit's allegations and said the men never raised the complaints to the company during their brief time at the plant.

"Given our size, we recognise that unfortunately at times there will be cases of harassment or discrimination in corners of the company. From what we know so far, this does not seem to be such a case."

The suit, filed in Alameda County Superior Court, claims Owen Diaz and his son, Demetric, were called niggers while they worked at the Fremont factory, and supervisors did little to stop it. A third man, Lamar Patterson, also claims he was subjected to insensitive racist remarks.