Eye-witness accounts suggest that a fault with a three million-litre tank of unleaded petrol at the Buncefield oil depot, near Hemel Hempstead, sparked the explosions.
The blast unleashed 300ft flames into the air and sent thick black smoke pouring across the town and beyond. The explosion, which injured 43 people, two seriously, shattered windows and ripped doors off hinges. The reverberations were felt up to 100 miles away in Europe.
The first blast at 6am was understood to have happened at part of the site which supplies Total and Texaco garages, and then spread to tanks containing kerosene for airliners and domestic fuel.
A chemical engineer for a major oil company said that he believed the only way an explosion of this size could have occurred in the cold weather was through a "huge and catastrophic leak of gasoline".
The enormity of the initial blasts prompted initial concerns, played down by police, of a plane crash or a terror attack on the depot, Britain's fifth largest which holds millions of gallons of fuel and supplies aviation fuel for Heathrow and Luton airports.
Fires raged for much of the day and explosions continued as around 2,000 people living nearby were evacuated, while police advised others to keep their windows and doors closed because of the thick plume of smoke rising from the flames.
Satellite pictures showed a dense mass dispersing east, west and southwards and by the afternoon a clear and sunny day forecast in London by meteorologists had turned gloomy.
Police said it was a "miracle" that no one had died and the low casualty list was because it was Sunday morning with few workers on-site and in the neighbouring industrial estate.
Last night around 100 firefighters were waiting to tackle the blaze but with sporadic explosions throughout the day they may have to wait until this morning, according to Hertfordshire's Chief Fire Officer Roy Wilsher.
Emergency supplies of foam concentrate were being delivered from as far away as Humberside. It will be mixed with water to create the millions of tons of foam needed to tackle the blaze. Mr Wilsher said: "This is possibly the largest incident of its kind in peacetime Europe. The damage a fire of this intensity will cause may, or may not, leave clues for the fire investigation team."
The blaze caused widespread disruption to traffic with the hazy skies causing minor delays at Heathrow and the closure probably until the end of today of the M1 motorway north of the M25 to junction 10.
In Hemel Hempstead around 300 evacuees spent the night in a leisure centre while schools, day centres and libraries in the town will be shut today. One fire fighter who was among the first crew ordered to enter the depot for an emergency rescue mission said the incident was one he had dreaded throughout his career.
"Before we went in for the search and rescue to the depot I contacted my parents because I didn't know whether I was going to come out.It's been one of these areas we know is at risk."
Phil Anderson, 34, the manager of the Leather Bottle, a pub in Leverstock Green, was frantic after the ceiling in his three-year-old son Charlie's room collapsed as he slept. He struggled to open his son's door. " Luckily he was unscathed. I was so shaky and he must have been the luckiest boy in the world," he said.
Norrie Vine, a 71-year-old taxi driver, was thrown from a chair in his flat in Wood Close, near Wood Lane End, Hemel Hempstead, by the blast.
He said: "It felt like the force of a 2,000lb bomb it was a pretty horrific thing. I was asleep in my chair when I nearly got blown out of my flat."
Employees' representatives last night complained that oil companies which use the site had cut costs to the bone and that there were only a tiny number of staff on duty at the time of the explosions.
One source said that only two maintenance workers were at the depot when the explosions happened.
Workers said most of the functions at the depots, including health and safety, had been contracted out to other firms.
Ron Webb, a national official at the Transport and General Workers' Union, which represents oil tanker drivers working out of the depot said it was a miracle that none of the drivers had been "brought out in body bags".
'There was a shard of glass on my pillow'
Catherine Sestak woke to find shards of glass flying towards her from her shattered bedroom window. Ms Sestak, a secretary in her fifties, was asleep in her bed when the first blast happened at 6am.
"The window collapsed and shards of glass came flying. There was a sharp piece of glass on my pillow inches away and glass covering my duvet," said Ms Sestak, who lives with her partner, George Swift, a lecturer, in Hunters Oak, half a mile from the oil depot.
She sustained minor cuts to her nose, but was otherwise unscathed. Most of her neighbours in the well-to-do suburb of Hemel Hempstead were left with broken windows, collapsed roofs and buckled front doors.
Ms Sestak described a mushroom-shaped black cloud with gigantic flames.
"It seemed like an image of those Kuwaiti oil refineries after the Gulf War. It was like seeing a film. We weren't sure what it was. We thought it could be a plane crashing or a petrol station up in flames," she said.
'The flames were as high as 200 feet'
Raheel Ashraf, 26, from High Wycombe, was working as a security guard at the building next to the depot, when the blast happened. Moments later he was standing on the top floor surrounded by a pile of rubble.
He had been working a night shift and was due to finish at 7am when he heard the explosion. Mr Ashraf described the scene that surrounded him as " like hell".
He said: "I was just taking a little tour in the offices and I was on the top floor. I could smell a lot of fumes and I had no idea where it was coming from. Within seconds, there was a blast. I fell down to the ground and when I opened my eyes there was nothing left of the building.
"It was a miracle I was standing there without a scratch on me. Luckily there was a lot of rubble, and I was able to jump down and crawl my way out of the car park."
Mr Ashraf said he was shocked by the devastation. "The flames were as high as 200 feet. I had to keep getting further and further away from the fire, which was out of control."
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