British police say they are treating the explosions, which left 39 people injured, as an accident although investigations are contnuing as firefighters battle to bring the blaze under control.

By late Sunday, a cloud of smoke had spread several hundred kilometres from the site at the Buncefield oil depot near Hemel Hempstead.

Health officials have said they do not believe the smoke is a serious health danger, although people with existing respiratory conditions have been warned to stay indoors.

The explosions and subsequent fire began shortly before dawn on Sunday and officials have said the blaze could burn for several days.

The initial explosion was felt as far away as France and the Netherlands.

Firefighters at the scene said the explosion was unprecedented in recent history.

"We have been informed by experts that this is possibly the
largest incident of this kind in peacetime Europe," said Roy Wilshire, Hertfordshire county's chief fire officer.

Most of the injured suffered cuts and bruises, said an official at a hospital treating blast victims.

One was seriously injured, having suffered lung damage from the impact of the blasts and was being treated in an intensive care unit at Watford General Hospital, the official said, adding he was on respiratory support.

However, he said the patient's condition was not critical.

"All indications at this stage are that this was an accident. However, clearly we will keep an open mind until we can confirm that for certain"

Frank Whiteley,
Hertfordshire police chief

Hertfordshire Police Chief Constable Frank Whiteley underlined previous police reports that the explosions, which occurred around 6am, appeared to be the result of an accident, although they were keeping an open mind. 

Just four days earlier an al-Qaida videotape appeared on the
Internet calling for attacks on facilities carrying oil
that has been stolen from Muslims in the Middle East. 

Luton Airport, a major air hub for London, mainly used by charter and low-cost flights, is about 16km away and there were initial rumours that a plane was involved, something police later dismissed.

As a result of operations at the scene, it was anticipated that there would be further explosions during the morning, and police wanted to reassure the public that this was nothing untoward, Whiteley said.

He advised people to stay away from the huge plume of smoke drifting over the area, saying: "It is an irritant, and would certainly make people who inhale it potentially cough, potentially irritate the eyes, and potentially feel nauseous."

London affected

Flames and smoke could be seen 
billowing into the sky

The blast was felt throughout a large part of London and the surrounding area.

Duncan Milligan, of Hemel Hempstead, said the blast woke him.

"There was a loud boom and the house shook violently," said Milligan.

"I am about three miles from where the explosion took place, but I can see flames high in the sky and smoke billowing everywhere. Police and emergency services are everywhere," he added.

The depot is jointly run by oil companies Total and Texaco. 

Motorway closed

Ian Hutchison, a spokesman for Total Oil, said two employees
who had been at the site were unhurt. 

Neighbouring commercial
buildings were damaged

"We have been able to contact our two members of staff and
they are both safe," he told BBC Television. 

Police closed the nearby M1 motorway in both directions, causing transport chaos, and warned against panic buying of fuel after witnesses saw long queues of cars at petrol stations. 

"There is nothing to suggest there will be a fuel shortage as a result of this," Chief Constable Whiteley said.

A Ramada hotel nearby was evacuated after the blast blew out some of its windows, with injuries reported.

An AFP reporter near the fuel depot saw damaged buildings,
splintered tree branches and debris thrown over country roads.

Windows were smashed and tiles blown off roofs.