Firefighters described the blaze at the Hemel Hempstead oil depot as an "environmental tragedy" last night as they battled to quell the inferno and contain the toxic cocktail of chemicals it was spewing out.
With worsening weather conditions sending the thick black cloud of smoke swirling around the site, the firefighting effort was suspended amid fears of a new explosion.
A blanket of dense fumes continued to hang over a large swath of the South-east despite the efforts of more than 150 firefighters from 16 fire services who had battled since first light to extinguish the blaze which has raged since dawn on Sunday.
Crews could get no closer than within 100 yards of the flames because of the heat. Unions warned that fire services were "desperately stretched" , while Hertfordshire's chief fire officer, Roy Wilsher, said his men were in "uncharted territory" as they fought post-war Europe's largest industrial fire. "It looks devastating. It looks like an apocalypse. Something people are just not used to," he said.
Mr Wilsher was forced to "pause" the operation amid fears a " highly volatile" fuel tank could explode. He played down the " temporary setback", but when the site was declared safe and firefighters returned in the evening, they tried to prevent further explosions by cooling the area, rather than tackling the blaze with foam.
As discussions continued as to how to continue fighting the fire, there were suggestions that it would burn itself out in 72 hours.
John Prescott, the Deputy Prime Minister, paid tribute to the bravery of those dealing with the fire. In a Commons statement he said the cause of the explosion was still not known, although the authorities continued to play down suggestions of terrorism, insisting that the most likely explanation remained an accident. The Health and Safety Executive will hold a full investigation, Mr Prescott said.
Firefighters began tackling the flames at 8.20am yesterday after agreeing a plan of action with environmental and oil industry experts. They used 32,000 litres of water and foam per minute to extinguish 12 of the 22 tanks that were alight. But late in the day fears were raised over the contents of tank 7.
Further safeguards were taken to ensure safety, including a temporary closure of the M1 between junctions 6 and 10, causing rush-hour chaos before it reopened last night. The M10 was closed all day.
Structural surveyors sent to the site to assess the damage yesterday morning were withdrawn.
The main environmental threat came from the toxic cocktail of chemicals produced when foam and water are sprayed on to a burning mix of fuel. The Environment Agency said barriers or "bunds" had been erected around the Buncefield oil storage depot to prevent toxic sludge from running off into water supplies.
Residents in the area were warned to stay indoors and local schools will remain closed today as a precaution against children breathing the smoke.
A spokesman for the Environment Agency said: "The fire waters, combined with oil and petrol, could have a severe impact on surface and ground water quality and in turn aquatic life. This pollution would occur only if the run-off escaped from the site, and we are working with the fire service to ensure measures are taken to avoid this."
Surface run-off from the firefighting operation is being collected and pumped into storage tanks. The local water company has closed abstraction boreholes in the area as a precaution against toxic substances contaminating drinking water.
Colin Chiverton, the environment manager at the Environment Agency, said his officials were working with the Health Protection Agency to assess the likely spread of the smoke plume and its impact on human health and the environment. "Based on the type of fuel stored at the site and the foam used we do not believe there is a significant long-term risk to the environment," he said. A spokesman for the Health Protection Agency said it was advising people in the Hemel Hempstead area is to stay indoors and keep their windows closed. "Air quality testing has not shown significant pollution and therefore the risk to health is low, but as a precaution residents are advised to follow the existing advice," he said.
"Those most at risk from inhalation of smoke particles are people with [an] existing respiratory problem such as asthma or chronic bronchitis or cardiac problems. They should be especially vigilant. If they see smoke or there are soot particles on windowsills or external surfaces they should stay indoors with windows and doors closed."
Neil Cope, an environmental scientist at the Centre for Hydrology and Ecology in Edinburgh, said the greatest risk was from the direct inhalation of soot and smoke which contains particles small enough to penetrate deep into the lung. "Some carcinogens can stick to the surface of soot particles and if they are small enough can easily get into the lungs," he said.
"However, my guess is that because this is refined fuel rather than crude oil, much of the more dangerous substances have already been taken out. "
An aircraft chartered by the Met Office flew across southern England yesterday to measure the size of the soot particles released from the fire and the extent of the toxic plume. Steve Ball, the head of the facility for airborne atmospheric measurements at the Natural Environment Research Council, said the data will be fed into a computer to try to model how the plume will disperse over the next few days.
Meanwhile, Hertfordshire Police tried to allay fears of lawlessness and looting in the exclusion zone area where homes lay abandoned and potentially vulnerable to burglars. Officers were reported to be patrolling residential areas and insecure premises on the industrial estate.
The explosions at the fifth largest fuel distribution depot in the UK have sparked panic-buying of petrol by motorists, despite reassurances from the oil industry that it had plans to deal with such an emergency.
Downing Street has urged drivers not to rush to the pumps but has acknowledged that there could be "short-term local difficulties". A spokesman for Tony Blair said oil executives and the Department of Trade and Industry were working together "to overcome any distribution issues".