Robbie Miller, operations manager for Uber's self-driving trucks, wrote to the firm's top executives saying the cars were "routinely in accidents".

This was partly down to faults with the technology and partly because of the "poor behaviour" of operators, he said.

Uber has yet to respond to the revelations.

However, it told news and information website The Information in a statement: "Right now the entire team is focused on safely and responsibly returning to the road in self-driving mode.

"We have every confidence in the work that the team is doing to get us there.

"Our team remains committed to implementing key safety improvements, and we intend to resume on-the-road self-driving testing only when these improvements have been implemented and we have received authorisation from the Pennsylvania Department of Transportation."

Mr Miller's email was sent to the head of Uber's autonomous vehicle unit, Eric Meyhofer, six other executives and lawyers.

Several of the drivers were not "properly vetted or trained", he wrote.

It was sent on 13 March, only days before a fatal collision in Tempe, Arizona in which an autonomous Uber car hit and killed pedestrian Elaine Herzberg.

Uber suspended all its tests following the accident, which is still under investigation by the National Transportation Safety Board.

In June it was revealed that the safety operator of the car was watching TV just before the accident.

In his email, Mr Miller wrote: "The cars are routinely in accidents resulting in damage. This is usually the result of poor behaviour of the operator or the AV (autonomous vehicle) technology.

"A car was damaged nearly every other day in February. We shouldn't be hitting things every 15,000 miles."

National Transportation Safety Board investigators have examined the vehicle involved in the crash

He went on to describe three incidents, including one where a car "drove on the sidewalk for several metres". In another, an accident was avoided only because a vehicle in the adjacent lane swerved to miss the Uber prototype, according to his email.

Mr Miller complained that such incidents were "ignored", with the logs left unreviewed by senior managers for days or even weeks.

"This is not how we should be operating," he said.

He recommended that Uber reduce its fleet size by up to 85%, stop tests after all accidents and review any incident, even minor ones "immediately".

Mr. Miller is currently working at a self-driving truck start-up led by Anthony Levandowski, who was accused of stealing secrets from Google's self-driving division Waymo.

Mr Levandowski was fired by Uber after he declined to testify in the lawsuit filed by Waymo against the firm. The case was settled in February with Uber giving a stake of its company to Waymo and agreeing not to use its technology in its self-driving cars.

Mr. Miller also worked at Mr Levandowski's prior company Otto, which was acquired by Uber, and worked for him at Waymo.