A huge fire which destroyed an Ocado warehouse was caused by a fault in a charging unit, investigators said.
Nearby homes were evacuated while firefighters tackled the blaze at the grocery distribution centre in Andover, Hampshire, over four days in February.
Ocado said an investigation by Hampshire Fire and Rescue Service and its insurers concluded the electrical fault caused a robot to catch fire.
It said it had taken steps to avoid it happening again.
Investigators found an electrical fault at a battery charging unit caused the plastic lid on the top of a grocery-carrying robot to catch light.
The online retailer said it had since introduced additional localised smoke detectors, removed the plastic lid on its robots and added heat sensors.
About 100 residents were moved out and a 500m exclusion zone set up amid fears a three-tonne cylinder of toxic ammonia gas might explode.
At the height of the fire, more than 300 firefighters worked to tackled the flames.
Four firefighters were treated for minor smoke inhalation, but no Ocado staff were injured.
More than 30,000 orders - 10% of Ocado's capacity - were processed by robots at the Andover warehouse each week.