Google's spin-off self-driving firm Waymo has said it won't build a self-driving car from the ground up again because it's a "distraction".

Speaking as the company prepared to open a Detroit factory which will be used to fit Chrysler and Jaguar cars with its self-driving software, Patrick Cadariu, its head of vehicle supply chain operations, said making cars was "really hard".

"Our goal is not to build cars. It's to build the world's most experienced driver. So that is our core business. There's other people that are very, very good at making cars, and they should do that, and we'd love to work with them," he said.

The firm, which was first created as Google's self-driving car company in 2009, has established a taxi service in Phoenix, Arizona, where it has also begun to operate some cars without back-up drivers.

Most self-driving car companies are taking a similar approach, by fitting existing cars with self-driving technology, but one of Waymo's Silicon Valley rivals, Zoox, founded in 2014, plans to build its own cars from the ground up.

Its co-founder Jesse Levinson has previously said that its approach "makes a lot of sense" because it can create a car specifically designed to be driverless.