Three companies were facing potentially unlimited fines today after they were found guilty of health and safety breaches in connection with the explosion at the Buncefield oil depot.
TAV Engineering Ltd, of Guildford, Surrey, was found guilty today of failing to protect workers and members of the public, the Health and Safety Executive (HSE) said.
Motherwell Control Systems 2003 Ltd, which is in voluntary liquidation, was found guilty of the same charge on Wednesday at St Albans Crown Court.
Hertfordshire Oil Storage Limited (HOSL) was also found guilty of failing to prevent major accidents and limit their effects - a verdict which could be reported for the first time today.
Today the company also pleaded guilty to causing pollution to enter controlled waters underlying the vicinity around Buncefield, contrary to the Water Resources Act, an HSE spokeswoman said.
The verdicts follow a joint prosecution by the HSE and the Environment Agency (EA) which the bodies today described as the "biggest and most complex criminal inquiry" they had worked on together.
The destruction at the Hertfordshire depot came after a massive vapour cloud ignited when 250,000 litres of petrol leaked from one of its tanks.
The blast on December 11 2005, widely thought to be the largest explosion in peacetime Europe, measured 2.4 on the Richter scale and could be heard 125 miles away.
During the trial, which started on April 15, jurors were told that the environmental damage caused was still not known but could last for decades.
Total UK has previously admitted three health and safety breaches in connection with the explosion, while the British Pipeline Agency Ltd had also admitted two charges.
Sentencing is due to take place at St Albans Crown Court on July 16, the HSE spokeswoman said.
She said fines in relation to breaches of the Health and Safety at Work Act could be unlimited, while an Environment Agency spokeswoman said the same applied for breaches of the Water Resources Act.
In a joint statement, the bodies said: "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."