Explosions at a fuel depot near Hemel Hempstead in Hertfordshire were heard in several counties.
Residents have been told to keep their windows and doors closed
Eyewitnesses, many of whom were woken by the noise just after 0600 GMT, have been describing their experiences.
Some were working in the vicinity of the depot at the time of the blasts.
DAVID KIRK, SECURITY FIRM DIRECTOR
Mr Kirk's company runs security on three premises on the industrial estate. He was arriving to relieve a colleague from the night shift when he witnessed the first explosion.
"One of our colleagues was in the reception area of a building across the road that took the full blast. Fortunately he bent his head down to write in the log book when the front of the building blew in - the glass went over his head.
"We were lucky that none of the six staff we had there were hurt. I was later than usual because I'd had to unlock another building on the way - had I been on time we would have been out in the car park doing the handover for the day."
JONATHAN BARR, WORKS NEXT TO FUEL DEPOT
Mr Barr, who works for a firm in the building next to the oil depot, said the blast knocked him to the ground leaving him with cuts and grazes.
He said he was sitting at his computer in his office "and the next minute I was on the floor and it was pitch black and I was covered in debris and smoke and dust everywhere. I was thrown off my chair."
"I didn't hear anything. I was just working and then I was on the floor.
TROY WOODLAND, SECURITY WORKER
Troy Woodland was close to the explosion
"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.
"I was aware of the fact that glass shards and bits of metal were flying past me as it pushed me across towards the wall."
PHIL SOFER, WHO WAS FLYING INTO LONDON DURING THE FIRST EXPLOSION
Phil Sofer, 29, from Peckham, south-east London, was flying back to the UK on a cargo aircraft from Japan.
"I was flying into Heathrow. I was looking out the window to watch the lights of London. As we made the final approach I saw this orange light., which I first thought was a fireworks display. But it developed into a huge ball of flame.
"We were just a little south of the Thames, we were circling so we were still quite high.
"I thought 'that is huge'. I realised it was a fire and grabbed my digital camera, and started shooting a movie clip.
"There were these giant flames dancing around and these smaller explosions.
"I was trying to orient myself and work out where it was. I realised it was north of London and thought at first it was Luton Airport."
IAN SILVERSTEIN, HEMEL HEMPSTEAD
Mr Silverstein was injured when the explosion damaged his house, which was close to the depot. "I woke up this morning to my house falling on top of me and my girlfriend in bed.
"My house was only 100 metres from the oil depot and I am not sure there is anything left. I am awaiting the police to let me go and see if there is anything left.
"All I know is my girlfriend and myself are very lucky to be alive. To anyone who knows me I am okay, a bit shaken and a few wounds, but okay."
Sylver Tamburrino, from Hemel Hempstead, had driven close to the Buncefield depot to pick up her partner who works in a nearby industrial estate only a street away from the where the explosion happened.
"I had just picked him up, one the dot at six. Usually he comes out at about 6.10, and I'm sitting waiting in the car. I don't know whether that was fate or not.
"We were driving along and I saw this white flash across the sky, and we thought it was lightning, and then it just went 'boom'.
"I can't even describe the explosion. My ears popped and the whole car vibrated.
"We turned back, because my partner is first-aid trained,a nd saw this huge column of smoke on the right-hand side. It just went op and out, almost like a mushroom cloud, but it was fire, not smoke.
"We could feel the heat coming through the windshield. We just turned back... it was a cold morning, We had the heating on in the car but we had to turn it off because the heat coming through the windshield was incredible."
NORRIE VINE, HEMEL HEMPSTEAD
Mr Vine, a 71-year-old taxi driver, was thrown from a chair in his flat when the blast happened, and thought a bomb had gone off.
"At first I thought it was an al-Qaeda thing, I have lived around the world and I know what explosions sound like," said Mr Vine, adding that it felt like the "force of a 2,000lb bomb".
"I was asleep in my chair when I nearly got blown out of my flat. There was just one huge explosion and everything shook.
"I am all right, I have just one broken window and insulation from the roof has come down because the explosion blew open the trap-door in the ceiling."
"It was terrifying. I thought that it was going to be the end of me, everything was shaking. I got lifted out of my chair and dumped back on the ground."
"I am going to stay here for now and have my cup of coffee."
YVETTE AND ED VAN HATTEM
"My wife and I live in Hook of Holland, on the south west coast of the Netherlands, approximately one mile from the shore.
"This morning, just after seven Dutch time, the roof of our house suddenly cracked. We were both awake and heard it. This alarming sound (there was no wind at all and everything was very quiet) was directly followed by two very heavy thunderlike sounds, obviously far from our home.
"After we watched Ceefax on the Dutch television we realised that we must have been witness of the shockwave and the sound of the blast of the explosions near London.
"Considering the fact that we live about a hundred miles from London we thought it might give some idea of the intensity of the explosions."
JOHN HEBBLETHWAITE, HEMEL HEMPSTEAD
"I was woken up by a huge noise and my wife's screams at 0605 GMT. We thought out boiler had blown up. I went outside to see what was going on and the sky was orange.
"We are one mile from the fire and the rear of our house is bathed in morning sunlight with a clear blue sky, whereas the front is grey and dark."
PAUL TURNER, TANKER DRIVER
Mr Turner had just arrived for work at the depot when the explosion happened.
"Before six o'clock I pulled up into the car park and I was walking in, ready to go into the building to get my truck to go out on the road and then I just saw this great big ball of fire come up behind the building.
"I still didn't know what it was at that point, it was about 50 metres wide, and then the loudest explosion, the loudest bang I've ever heard in my life.
"It took me off my feet. I'm not quite sure whether my knees had buckled or whether the blast had taken me off my feet, I was just in shock at that point.
"I got up, turned around and just ran as fast as I could straight to my car, which when I think about it now was still in one piece, jumped in my car and sped out of the building as fast as I could."
Police have confirmed there were casualties at the oil depot
"I was working with a colleague and once we got up we managed to find each other, we had to break a window with a chair to get out because we couldn't open any of the doors they were all bent up.
"So we managed to stumble down the stairs, what was left of them, and get out to the front of the building. And then luckily there was people driving up and down in the cars and two people stopped and took us into took us to the hospital."
Ms Woodcock described how the curtains were sucked into the room.
"My first instinct was to look to see what had caused it. One of my children was screaming. The flames aren't as high now, the width is hard to tell now because the sun has risen. One side of the sky is blue, the other grey."
"I just heard this very long rumbling explosion which made us all think it was an aeroplane.
"The whole house was shaking and the cupboards all blew open.
"We ran out of the house just as the police were arriving on the scene - we could see the flames shooting up to 100 to 200ft in the air."
IAN CLARKSON, NORTH HARPENDEN
"At about six o'clock we were woken up by a huge noise - it was almost like thunder, but the whole house shook - dived out of bed, ran to the front window and saw flames, you know, very, very high and lots of cloud and smoke.
"Initial thoughts were that it was a plane crash, we're only about two to three miles away from Luton Airport, so I drove up, and the police had started closing off the motorway."
Mr Franklin, who lives just over half a mile from the depot, said he was woken up by "an absolutely massive loud bang".
"It actually broke two windows in the flat above us. There are just flames everywhere. Orange flames. The whole sky has just turned orange and black."
BBC JOURNALIST SINEAD WILSON, HEMEL HEMPSTEAD
"We instantly jumped out of bed and ran into the street and lots of our neighbours had done the same sort of thing. All the car alarms in the street were going off.
"As I look above me there's a huge plume of smoke basically covering the sky, there is a bit of a smell of gas in the air and lots of fire engines, lots of police cars in the surrounding area."
DUNCAN MILLIGAN, HEMEL HEMPSTEAD
Mr Milligan, who works for the Fire Brigades Union, said he was awoken by the explosion as his house "shook".
"There was a loud boom and the house shook violently.
"I am about three miles from where the explosion took place but I can see flames soaring high in the sky and smoke billowing everywhere."
DOMINIC AND SHEILA GIZZIE, HEMEL HEMPSTEAD
Mr Gizzie said: "We got woken up this morning with a huge explosion. We have a lot of damage, our fireplace was blown out, several doors came off and the garage door has broken."
"The flames were 100ft into the air, I have never seen anything so big."
The explosion was heard as far away as Holland
Mrs Gizzie added: "I have never heard such a loud explosion in my life, we did not know what it was, we have recently come back from New Zealand so we thought it may have been an earthquake.
"We were so shaky this morning, we had such a fright - you do not expect to wake up and have an explosion at your door. We just don't know what is going to happen."
HELEN GLASS AND DAUGHTER MEGAN , HEMEL HEMPSTEAD
"There was just a massive explosion and a long rumbling sound - at first I thought it was a gas explosion or that a plane had crashed.
"Both children came running into my room and were absolutely terrified.
"We just have to sit here because we don't know whether we will go back tonight or tomorrow - I hope it is tonight.
"I am worried about my house but there is not a lot we can do at the moment, if it is going to happen it is going to happen.
Daughter Megan, nine, said: "I was really shocked and scared by what we saw - the radiator in my bedroom fell off the wall and a mirror cracked."
CHRIS OLD, CHAULDEN
"I was woken at about 6am by a loud noise, and at first I thought it was just a roll of thunder. Obviously it wasn't. I went downstairs and saw what was going on - I live about two miles away from the incident.
"I received a text message from my friend who lives in Leverstock Green, just describing what was happening. I quickly went up there in my car.
"I saw one of his neighbours, he was quite traumatised. He had a cut eye and a scar on his face. I don't know what had happened to him. They are about half-a-mile away from the oil depot. I drove him to the hospital
"I must have been there a couple a minutes before the emergency services arrived. There weren't many people around. After that, when I came back it was chaos. Everybody was out of their houses."