Californian regulators have taken aim at ride-sharing car operators, claiming that three of the main companies in the state engage in "unlawful business practices".

Concerns over the background checks carried out on drivers by Uber, Lyft and Sidecar have led district attorneys in San Francisco and Los Angeles to write joint letters to the firms, warning them they may look to halt their operations, the Financial Times reports.

They stated that the smartphone app-enabled services constituted a "continuing threat to consumers and the public" and that  they could face "injunctive relief and civil penalties" if they failed to amend the claims that they make to customers about checking drivers' criminal records. 

Traditional taxi services in San Francisco have been harmed by the emergence of new app-based services, which allow anyone with a car, a smartphone and a clean driving record to pick up passengers.