The first of several explosions hit the Buncefield fuel depot just after 0600 GMT on Sunday, 11th Dec 2005, injuring 43 people, two seriously.
Witnesses said another two explosions followed the first at 0626 GMT and 0627 GMT at the site near junction 8. The fire, which police believe was an accident, could burn for another day.
About 2,000 people living near the site have been evacuated, while police have advised others to keep their windows and doors closed because of fumes.
With light winds blowing to the south-east and south-west, thick clouds of smoke is likely to be spread over large parts of southern England by Monday.
The M10 motorway is closed in both directions between junction 1 and junction 7 as well as some arterial roads in Hemel Hempstead.
Motorists have been told not to go "anywhere near the M1 from the M25 upwards". Hertfordshire police said about 70 schools in the Hemel Hempstead and St Albans areas would also be closed on Monday.
At Heathrow airport some flights were forced to delay landing because of smoke, but Luton airport was operating as usual.
Police said there was no indication the explosion would cause fuel shortages and warned against panic-buying. The M1 finally reopened around 2200 GMT on Sunday after being closed for 12 hours. Rail links through Hemel Hempstead were unaffected.
Hertfordshire's Chief Fire Officer Roy Wilsher said: "The damage a fire of this intensity will cause may, or may not, leave clues for the fire investigation team." Clues that could explain how the blasts happened may have been devoured by the fire.
A security worker at the depot described what happened. "I sat down and all of a sudden there was a huge orange light and a massive explosion which blew the doors through and knocked me off my chair, and the ceiling fell in".
In total, 20 petrol tanks were involved, each said to hold three million gallons of fuel. Many houses have been damaged, with some reporting feeling effects from the explosion as far away as Oxfordshire - while it was heard in a number of counties and even France and the Netherlands. Eye witnesses reported buckled front doors, cracked walls and blown-out windows.
Fire chiefs are consulting oil industry experts about using millions of litres of foam to quell the blaze. Meanwhile samples of smoke are being taken to determine the long term effects of exposure, if any, according to Dr Jane Halpin, director of Hertfordshire Public Health.
The Buncefield fuel depot which was rocked by explosions on the morning of 11 Dec '05 is co-owned by global oil giants Total and Texaco. The official name of the core Total-Texaco facility at the site is Hertfordshire Oil Storage Terminal.
Located near Hemel Hempstead in Hertfordshire, England, it supplies petrol and other oil products to the wider South East and London regions and provides aviation fuel for a number of airports, including nearby Luton and Heathrow.
Although Buncefield is a main facility, petrol firms and the airports insist there will be no supply shortages.
First opened in 1968, it employs 16 people and is the fifth largest such depot in the UK. It handles some 2.37 million metric tonnes of petrol and other oil products a year, filling 400 tanker lorries per day.
The petrol and other products are pumped to the facility by an underground pipeline from the Lindsey Oil Refinery on Humberside. This pipeline was completed in 1990.
A further pipeline then continues directly from Buncefield carrying aviation fuel to Heathrow. The wider Buncefield site is also used by BP, Shell and British Pipeline.
French-owned Total is the world's fourth largest oil company. Texaco is part of US group Chevron, the fifth biggest.
The Buncefield depot feeds filling stations for diesel and petrol, industrial oil and domestic heating oil for north London, Hertfordshire, Buckinghamshire and Northamptonshire.
