Three companies have been convicted of health and safety breaches in relation to the Buncefield oil depot explosion in Hertfordshire in December 2005.
TAV Engineering Ltd, of Guildford, Surrey, was found guilty earlier of failing to protect its employees.
Hertfordshire Oil Storage Limited and Motherwell Control Systems 2003, Liverpool, were convicted of breaches at St Albans Crown Court on Wednesday.
The blast injured 43 people and homes and businesses were destroyed.
The explosion, which could be heard 125 miles away, happened when a massive vapour cloud ignited after 250,000 litres of petrol leaked from a tank, the court heard.
Motherwell Control Systems 2003 Ltd, which is in voluntary liquidation, was found guilty of failing to protect its employees.
HOSL was found guilty of failing to prevent major accidents and limit their effects - a verdict which could not be reported on Wednesday for legal reasons.
The company pleaded guilty on Friday to causing pollution to enter controlled waters underlying the vicinity around Buncefield, contrary to the Water Resources Act.
TAV Engineering Ltd had denied committing a breach between 1 October 2003 and 12 December 2005.
Total UK and the British Pipelines Agency Limited had already pleaded guilty to offences under the Health and Safety at Work Act.
The verdicts follow a joint prosecution by the Health and Safety Executive and the Environment Agency (EA).
They said in a joint statement: "This was the biggest and most complex criminal inquiry we have worked on together - the product of many hundreds of hours of painstaking forensic investigation.
"When companies put workers and members of the public at risk and cause environmental damage we will prosecute.
"When the largest fire in peacetime Europe tore through the Buncefield site on that Sunday morning in December 2005, these companies had failed to protect workers, members of the public and the environment.
"The scale of the explosion and fire at Buncefield was immense and it was miraculous that nobody died. Unless the high hazard industries truly learn the lessons, then we may not be that fortunate in future."
Sentencing of the companies by trial judge Mr Justice Calvert-Smith will take place back at St Albans Crown Court next month.