Investigators looking into last year's huge explosion at the Buncefield oil depot north of London said today it was probably caused by the ignition of a vapour cloud after a storage tank overflowed.
An automatic safety system should have prevented the overflow but had failed to cut off the fuel supply, they added.
A series of explosions at the depot in Hemel Hempstead early on December 11th led to a three-day fire that created a pall of black smoke over a swathe of southern England.
In a report, the Health and Safety Executive said a tank had been overflowing for more than 40 minutes before the blast.
But it said the extensive damage from the explosion meant it had not been possible to identify what sparked the cloud of fuel vapour that would have formed.
Possible candidates were a nearby emergency generator, a fire pump-house or even a spark from a passing car.