A fire that destroyed an Ocado warehouse spread because a detection system failed and staff turned off the sprinklers, a report has found.
The distribution centre for the online food retailer in Andover, Hampshire, burned for four days in February.
Hampshire Fire and Rescue Authority's report found there was an hour's delay in dialling 999 and staff initially tried to tackle the blaze themselves.
Ocado said it was undertaking a "thorough examination" of the causes.
Earlier this month the firm, which aims to rebuild the facility, said the fire had cost it £100m.
It plans to cut 400 jobs as a result of the blaze, which investigators said was caused by an electrical fault in a battery charging unit which caused a plastic lid on top of a grocery-carrying robot to catch light.
The latest fire service report found the fire started at 01:41 GMT on 5 February but the fire detection system "did not detect the fire as designed" so it was first spotted by an engineer at 02:15.
The sprinkler system started operating 11 minutes later but was then turned off by Ocado engineers for five minutes which led to a "significant" growth in the fire.
Once staff realised the fire was not being extinguished they turned the sprinklers back on and finally dialled 999.
The report added early intervention "was not achievable" due to the delay. It added the "system failure and human factors" were "contributory to the eventual outcome".
A full report is due to be published in September.
More than 30,000 orders - 10% of Ocado's capacity - were processed by robots at the warehouse each week.
The company previously said it has introduced additional localised smoke detectors at its other centres, removed the plastic lid on its robots and added heat sensors.
It also said "all efforts are being made to minimise the disruption to consumers", including by opening a temporary centre in Andover.