Oil giant Total UK today admitted health and safety breaches in connection with the massive Buncefield oil depot explosion.
It entered written pleas of guilty to two charges under the Health and Safety Act, and one of polluting water under the Water Resources Act.
Four other companies, Hertfordshire Oil Storage Ltd, British Pipeline Agency Ltd, TAV Engineering Ltd and Motherwell Control Systems 2003 Ltd, pleaded not guilty to breaking health and safety laws.
Total is not expected to be sentenced until trials are held for the other companies.
The case at the Old Bailey was brought by the Health and Safety Executive and the Environment Agency.
A series of explosions, including one major one, ripped through the depot in Hemel Hempstead, Hertfordshire, on 11 December, 2005.
Widely thought to be the largest ever explosion in peacetime Europe, it measured 2.4 on the Richter scale and could be heard 125 miles away.
It left 43 people injured and forced 2,000 others to leave their homes.
The disaster occurred after the spillage of 300 tonnes of petrol from the top of one of the tanks at the site.
A huge vapour cloud formed and ignited, sending a giant plume of black smoke over Hertfordshire and much of south-east England.
Buildings on the surrounding industrial estate - and some homes up to three miles from the scene - suffered severe structural damage.
The depot held massive stocks of not only oil and petrol but also the aviation fuel kerosene, used to supply airports across the region including Heathrow and Luton.
Some 78 recommendations were made later by the Major Incident Investigation Board.
The board's chairman, Lord Newton of Braintree, said the decision to prosecute the five companies was "an important milestone".
In March this year, the High Court decided that Total should pay the property damage bills of individual and business claimants - a decision due to be appealed against next year. Claims amounted to £750 million.
A trial was provisionally fixed for April 14 next year at St Albans Crown Court.
Total admitted exposing staff and members of the public to risk, and to allowing water below the depot to become polluted following the blast.