A storage tank was overflowing for more than 40 minutes prior to the Buncefield oil depot explosion in December 2005, a report said today.
Unleaded petrol was being piped into the tank at the depot in Hemel Hempstead, Hertfordshire, for 11 hours prior to the blast on December 11.
But an investigation report said the tank would have been full by 5.20am - 41 minutes before the explosion.
The report follows an investigation set up by various agencies - including the Health and Safety Executive - following the explosion.
It has been produced by the Buncefield Major Investigation Board chaired by Lord Newton.
The investigation board says the report focuses on the "physical processes" but does not seek to address any shortcomings in the design and operation of the Buncefield plant "in order to avoid prejudicing future legal considerations".
But it said investigators have identified the tank from which fuel escaped prior to the blast.
"The investigation has identified that it was tank 912... from which the fuel escaped," said the report.
"This tank started receiving unleaded petrol from the pipeline from Thameside at around 7pm on the evening of December 10."
The report added: "Calculations show that the tank would have been full by about 5.20am, overflowing thereafter.
"The separate high level protection system which should have automatically closed the valves to prevent overfilling did not operate.
"Between 5.20am and 6am the fuel would have flowed from eight breather holes in the roof of the tank."
It went on: "Seismological evidence shows the main explosion occurring at 06.01:32. Eyewitness accounts also describe the main explosion being followed by a number of smaller explosions."
The report said a large vapour cloud probably developed over the site, but it added: "Because of the extent of subsequent damage it is not possible to state with any certainty what ignited the vapour cloud."
The report said the most likely "candidates" were a generator in a nearby car park and a pump-house at the depot.
The report said investigators found an "anomaly" in a hi-tech system designed to monitor the levels of fuel in tanks.
"Tank 912 was fitted with instrumentation that... measured and monitored levels and temperatures of the liquid in the tank," said the report.
"The instruments were connected to an automatic tank gauging system in common with all the other tanks on the site.
"Tank levels were normally controlled from a control room using the automatic tank gauging system."
It adds: "Examination of the records for tank 912 from the automatic tank gauging system suggests an anomaly."
The report said an alarm should have been triggered when the level of liquid in the tank reached its maximum.
"The tank also had an independent safety switch which provided the operator with a visual and audible alarm in the control room when the level of liquid in the tank reached its specified maximum level," the report said.
"The alarm also initiated a trip function to close valves on the relevant incoming pipeline.
"The ultimate high-level safety switch on the tank sensed when the liquid reached its specified maximum level should all other alarms and controls fail to prevent this."
But the report added: "Automatic shutdown did not take place."
The report revealed that the pipeline was delivering fuel at a rate of 890 cubic metres per hour in the 40 minutes before the blast.
Despite a number of safety features including warning lights, alarms and automatic shut-off valves, the overflowing tank was not noticed.
One of the safety features, the "ultimate high level safety switch", should have sensed when the fuel reached its maximum level and provided an alarm to operators in the control room as well as initiating the automatic shutdown of fuel delivery.
"At the time of the incident, automatic shutdown did not take place," the report said.
The blast, investigators believe, could have been ignited by a number of sources including the fire pump-house on the HOSL West site as well as an emergency generator cabin in the neighbouring office car park or, possibly a spark from passing cars.