Police and emergency crews raced to the scene of the blasts which rocked Buncefield fuel terminal, near Hemel Hempstead, in Hertfordshire, around 6am.
There were unconfirmed reports of casualties at the depot. Initial reports said witnesses heard a plane going overhead shortly before the first explosions - but police were quick to dismiss that suggestion, insisting the incident was an "accident".
The fuel terminal, which is close junction 8 of the M1 forced the closure of the motorway in both directions between junctions 6 and 10. The M10 was also completely closed.
The first blast happened just after 6am at the fuel terminal in Leverstock Green, which is believed to supply aviation fuel via pipes to Heathrow airport.
The force of the blast - which was heard up to 40 miles away - sent flames shooting 200ft into the sky.
A spokeswoman for Hertfordshire Police said: "Police and other emergency services are attending the area of Buncefield oil depot near Hemel Hempstead this morning following reports of an explosion just after 6am.
"There is nothing to suggest at this stage that this is anything but an accident, but we are keeping an open mind.
"Contrary to rumours in circulation, there is also nothing to suggest that a plane was involved in this accident."
Witnesses said homes and surrounding businesses were damaged by the force of explosions,
Residents nearby said they were awoken by a "loud boom" as their houses "shook".
People living as far away as Surrey reported that they heard the initial blast. Two smaller blasts followed at around 6.20am.
It is understood that windows of the Ramada Hotel, in nearby Hemel Hempstead, were blown out and a number of people were thought to have been injured.
The hotel, along with the neighbouring Holiday Inn, was evacuated.
A Holiday Inn guest told Sky News: "We were in our room and at about 6am there was this huge explosion. The room shook."
Witness Paul Hayes said he was close to the scene of the blast on an industrial estate.
He told Sky News: "I heard alarms going off. Buildings were smashed and falling down. Someone told me a petrol dump had exploded."
Duncan Milligan, of Hemel Hempstead, said he was awoken by the explosion as his home "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.
"There is clearly a building on fire near the motorway and police and emergency services are everywhere."
An unnamed woman told Sky News: "My house was shaken. CDs were falling off the racks. I jumped into my car and we went towards the industrial estate.
"I could see buildings with windows smashed. I could feel the heat from the flames.
"Emergency services were there and people seemed to arrive from nowhere. There was concern about people living in cottages nearby. A bungalow is thought to very close. People are being evacuated."
There were reports that drivers had pulled off the nearby M1 as they saw the flames.
Flights at Luton airport - about 10 miles away - in Hertfordshire were operating normally.
An employee at Buncefield told BBC News 24 that two workers were at the site overnight and were both uninjured.
He said: "The terminal is a joint venture between Total and Texaco. Two people were on the site but they are reported as safe."
Gary Sanderson, an ambulance worker told Sky News 120 people had been evacuated from a hotel in Hemel Hempstead.
He said: "Contingency plans are in place. It's very hectic at the moment."
At the site of the explosion, huge flames at least 40ft high continued to burn sending a vast plume of black smoke miles into the sky.
Chris Fraser, 55, an engineering manager, said he thought a plane had crashed nearby.
He said: "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.
"There was then at least another four explosions."
Houses on Wood Lane End, immediately next to the scene of the blast, were all evacuated.
Locals said they had heard reports of front doors buckling, leaving residents trapped inside.
Comments