Images of smoke and blasts
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Deputy Prime Minister John Prescott has visited the scene of the blasts which injured 43 people, two seriously.
The fire chief described the incident at the Buncefield fuel depot near Hemel Hempstead, after 0600 GMT, as possibly the largest in peacetime Europe.
The M1 finally reopened around 2200 GMT after being closed for 12 hours.
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."
Junction 8 of the motorway, aswell as some slip roads, however, remain closed. The Highways Agency said it would review the situation on Monday morning.
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.
Thick clouds of smoke are continuing to spread to the south-east and south-west of the site.
One person admitted to Watford General Hospital in intensive care with respiratory problems has now been stabilised. Another person is in Hemel Hempstead Hospital being kept under observation.
The other 41 people were treated for minor injuries and discharged.
Witnesses said another two explosions followed the first at 0626 GMT and 0627 GMT at the site near junction 8.
A security worker at the depot, Troy Woodland, 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," he told the BBC.
Further explosions
In total, 20 petrol tanks were involved, each said to hold three million gallons of fuel.
Your experiences from the blast zone
In pictures
But Chief Con Whiteley said there was "nothing to suggest anything other than an accident".
On his visit to the site Mr Prescott praised the response and offered government help but was told the emergency services were able to cope.
Hertfordshire's Chief Fire Officer Roy Wilsher said: "This is possibly the largest incident of its kind in peacetime Europe."
Fire chiefs are consulting oil industry experts about using millions of litres of foam to quell the blaze.
But Mr Wilsher said they needed to know they had enough foam before they began.
The BBC's Gavin Hewitt said about 100 firefighters were waiting to attack 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 blast was heard as far away as Holland
She said: "However, what I would restate is that those people who are most at risk are those people who have inhaled the smoke."
Tanker driver Paul Turner said he ran for his life after the explosion lifted him off his feet.
"I just saw this great big ball of fire come up from behind the building. It was about 50 metres wide," he told the BBC.
"Then there was the loudest explosion I have ever heard in my life. I got up, turned around and ran to my car and sped out of there as fast as I could."
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.
School closures
Eye witnesses reported buckled front doors, cracked walls and blown-out windows.
Of the 2,000 people evacuated 290 people have gone to a leisure centre while 50 others have been offered bed and breakfast accommodation.
HAVE YOUR SAY We heard an explosion and the whole house shook
Anil Taank, Northwood, Middlesex
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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.
The Buncefield depot is a major distribution terminal operated by Total and part-owned by Texaco, storing oil, petrol as well as kerosene which supplies airports across the region, including Heathrow and Luton.
The country's fifth largest fuel distribution depot, it is also used by BP, Shell and British Pipeline.
Police said there was no indication the explosion would cause fuel shortages and warned against panic-buying.
A spokesman for Total said: "We are doing everything we can to support the emergency services and to bring the situation under control."
A spokesman for the Department for Trade and Industry said it was too early to say what the effect would be on fuel supply but oil companies were getting oil from other parts of the south east and across the UK.
A spokesman for the Health and Safety Executive said it would be investigating the incident.
Concerned relatives can call a police casualty bureau on 0800 096 0095, or from abroad on 0207 1580125.
Location of Hemel Hempstead within the UK
A shot of the fire taken near the depot

A series of large explosions have occurred close to Hemel Hempstead  Hertfordshire, UK. The source of the explosions has been confirmed as the Hertfordshire Oil Storage Terminal (HOSL), Hemel Hempstead, known locally as the Buncefield complex. Up to  150 fire fighters are reported to be at the scene with 10 fire appliances and 1 specialist foamer.

The first 'blast' was heard near Hemel Hempstead on Sunday 11 Dec at 6 am. Further smaller explosions followed at 6:24am , 6:26am, 6:30am. BBC News 24 reported an additional, fourth large explosion.  Hertfordshire Police Constabulary are currently treating the explosion as an accident.  

Reports say the explosion, which registered 2.4 on the Richter scale, was heard as far away as Oxford, and Whitehall, Central London which is 60km (38 miles) away. Eyewitness statements report that the explosion was heard from at least 160km (100 miles) away and as far away as France and The Netherlands. Pilots reported noticing the blast from the North Sea and the West Country area of the UK.  The M1 motorway which runs close by has been closed in both directions near the blast which is causing travel chaos as other roads become congested.

Malcom Stewart, a BBC News24 eyewitness who is a tanker driver for the site has reported that the site supplies several oil companies and is a joint operation between Total UK and Texaco, it is also used by BP, Shell and the British Pipeline association. The complex is not a refinery but a storage facility for refined petroleum awaiting distribution to airports and filling stations. The eyewitness reports that the depot has approximately 20 tanks which can hold about 3 million gallons (11 million litres or 70,000 barrels) each.  Another News24 eyewitness has just reported that he has seen at least 5 of these tanks on fire.

Satellite image of Hemel Hempstead fuel explosion showing black smoke from the explosion near London

The depot operates on a 24 hour basis and is split into 2 parts - aviation fuel and domestic fuel.  A number of eyewitnesses have reported on UK news that the aviation fuel side appears to be the part of the site that has been affected.

Local authorities were not immediately available for comment but there have been reports of casualties.

Some reports on live television state that, "Several other neighbours said they did see a plane go into the depot."  BBC News 24 were also discussing the idea a possible plane crash as the cause of the explosions.  Hertfordshire police have now gone on the record to say that there is no plane involved (BBC News24).

The police have issued a contact number 0800 096 0095 and asked that people do not call the emergency services in Hertfordshire directly unless it is an emergency. 
Buncefield Fire, taken from Dunsmore, Bucks - about 20miles away.

In addition to being an oil storage depot, it is a major hub on the UK oil pipeline network with pipelines to Killingholme Lindsey Oil Refinery (LOR), Humberside (10 inch), Merseyside (10 and 12 inch), Coryton on the Thames Estuary (14 inch) and Heathrow (6 and 8 inch) and Gatwick airports radiating from it.

The disaster is believed to be the worst explosion at a petrochemical plant in the UK since the Flixborough disaster of 1974.
Hertfordshire's Chief Fire Officer Roy Wilsher said: "This is possibly the largest incident of its kind in peacetime Europe." 

A firefighting press officer said that they are stock piling foam from neighboring regions for a prolonged attach which they hope will stop the spread of the fire, however, the inferno itself will have to burn out which could take between 24 hours and a few days.

Despite the authorities saying that there is no need to panic buy petrol, filling stations have had above average queues since this morning and some small garages have ran out.

Deputy Prime Minister John Prescott has visited the scene.

Smoke Viewed from St Albans