THREE companies face huge fines after being convicted of involvement in the explosion which destroyed the Buncefield oil storage depot and surrounding homes and businesses in 2005.

The Health and Safety Executive brought charges against five companies, three of whom – TAV Engineering of Guildford, Motherwell Control Systems of Liverpool and Hertfordshire Oil Storage Limited – pleaded not guilty to charges relating to the blast just outside Hemel Hempstead but were convicted.

Two others, Total UK of Watford and British Pipeline Agency of Hemel Hempstead, admitted charges relating to health and safety issues and pollution at the depot. Hertfordshire Oil Storage, which had faced a second charge of causing pollution of controlled waters and faced another trial, changed their plea to guilty at St Albans Crown Court.

Mr Justice Calvert-Smith, who will sentence the companies in July, told the jury that the case had generated 6,000 pages of evidence.

The trial began in April when the jury heard that it was miraculous that no-one was killed when the depot exploded just after 6am on Sunday, December 11, 2005.

The blast was caused by spray from a quarter of a million litres of petrol that had spilled from a large tank at the site.

Prosecutor Andrew Langdon said the damage was greater because bunds – the walls surrounding the tanks to protect the environment – were not maintained and the chemicals poured into the ground below.

He described how in the early hours of the day of the explosion, a gauge became stuck as Tank 912 was filled. The tank continued to fill up and shortly before 4am it had reached a level when a cut-off switch should have been activated but it was not.

The level continued to rise and shortly before 6am, unleaded petrol flowing into the tank started to come out of the roof vents.

Supervisor Graham Nash, who had no idea earlier that anything was wrong, saw the white mist which resulted from the fuel pouring into the cold December night and hit the fire alarm which was probably the spark that ignited the low-lying vapour cloud.

Terminal supervisor Mark Forde said the explosion started with an “almighty bang” throwing him and four drivers to the ground. He looked at the devastated control room thinking Mr Nash must be dead but saw him on the phone.

Mr Nash said: “I looked out of the control room window and saw a massive low-level explosion that spread across the complete back width of the terminal.

“This was getting bigger and closer... towards the control room and was followed by a whooshing type noise and a long huge bang. The windows of the control room were blown in and I was thrown across the room.”

Tanker driver Terrence Hine described the build up of mist before the explosion as like “brain overload”. The winter air went hot and there was a huge orange fireball that left him unable to breathe.

Although no-one was killed in the blast, 45 people were hurt in the explosion and the owners of damaged residential properties around the site, a number of whom who live in the St Albans district, have spent years fighting for compensation.

Simon Grover, of St Albans District Green Party, commented: “I welcome this latest verdict although it will be little comfort to the many people affected by the explosion.

“This terrible event and the BP oil spill show some of the many problems with our addiction to oil.”