Firefighters are preparing to douse a ferocious blaze caused by a series of explosions that tore through an oil depot in Hertfordshire.
Flames reaching into the sky were seen from miles away
A vast plume of choking smoke is still billowing from Buncefield fuel depot, near Hemel Hempstead.
The first of several explosions hit the site just after 0600 GMT on Sunday, injuring 42 people, two seriously.
Clues that could explain how the blasts happened may have been devoured by the fire.
Firefighters were given the go-ahead to tackle the blaze on Monday morning following concerns about water pollution.
They had planned to start creating a "foam blanket" to smother the flames at midnight but this was postponed amid fears it could affect ground water supplies.
Police said the environmental concerns had now been satisfied and that the bid to douse the fire, using 250,000 litres of foam, mixed with 25,000 litres of water per minute, would start shortly.
A specialist crane has been brought in to assist with the operation.
Water will be pumped from the Grand Union Canal, two miles away, using high-pressure hoses.
On Sunday, firefighters worked to contain the fire, using a "curtain of water" between the flames and the remaining unexploded fuel tanks.
Seven out of more than 20 tanks remained intact. Each was said to hold three million gallons of fuel.
Meanwhile, the necessary materials and equipment for Monday's attack on the blaze were gathered from around the country.
An inquiry into the cause of the inferno by the Health and Safety Executive cannot take place until the site has been made safe.
Police officers - including anti-terrorist detectives - are investigating, but say there is "nothing to suggest" the fire was anything other than an accident.
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.
"This is possibly the largest incident of its kind in peacetime Europe."
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.
People with existing chest complaints are most at risk, along with those who breathed in large quantities of smoke.
About 2,000 people living near the site were evacuated, but some have been able to move back in.
Others have been advised to keep their windows and doors shut.
One person admitted to Watford General Hospital in intensive care with respiratory problems has been stabilised. Another person in Hemel Hempstead Hospital was being kept under observation.
Most of the other people treated suffered minor injuries and were discharged.
On the roads, the M1 reopened around 2200 GMT on Sunday but other roads remain closed at the situation is being reviewed.
Hertfordshire police said about 70 schools in the Hemel Hempstead and St Albans areas would also be closed on Monday.
Deputy Prime Minister John Prescott visited the scene of the blasts as survivors told of their escape.
A security worker at the depot, Troy Woodland, said: "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."
'Great ball of fire'
Tanker driver Paul Turner said he ran for his life after the explosion lifted him off his feet.
Smoke from the fire chokes the sky
"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 the initial bang was heard as far away as the Netherlands.
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.
BUNCEFIELD FUEL DEPOT BLASTS
With light winds blowing to the south-east and south-west, smoke is likely to be spread over large parts of southern England by Monday