About 36 hours after huge pre-dawn explosions Sunday at the Buncefield depot sent dark smoke billowing over southeast England and drifting towards France, firefighters had extinguished flames at 12 out of 20 burning storage tanks, using a "blanket" of foam and water.
They delayed their work on Monday in order to satisfy concerns they would not cause pollution to ground water around the depot near Hemel Hempstead, 40 kilometres from the British capital.
More than 150 firefighters fought the blaze until the afternoon before they suspended work over fears one burning tank contained volatile substances and could explode, Hertfordshire Chief Fire Officer Roy Wilsher said.
"Until we are absolutely sure what is in that tank we have withdrawn our firefighters," Fire Officer Wilsher said. "The last thing we want is human tragedy."
Fire Officer Wilsher was relieved with "better than expected" progress after having warned earlier that his crews were in "unchartered territory" as Britain and its European neighbors had never dealt with such a massive inferno before.
Strong winds
The job was all the harder as firefighters had to contend with swirling winds.
Late on Monday, however, a police spokeswoman said: "Fire crews received aerial imagery of the site which indicated where it was safe to resume operations. This work is likely to continue overnight."
She said that "cooling patrols" were taking place again at the site, although teams are unlikely to resume their efforts to extinguish the fire until Tuesday morning.
The inferno, which left 43 people injured but, astonishingly, no fatalities, is expected to take an estimated 72 hours to burn itself out.
It appeared to have started accidentally, although the cause remained unknown and clues as to what might have caused it were likely to be consumed in the inferno, police said.
As well as environmental concerns, fire chiefs had to wait for around 250,000 litres of foam concentrate to be sent from around the country before beginning the mammoth task of tackling the blaze.
About 25,000 litres of water per minute were being pumped from the nearby Grand Union Canal to be mixed with the concentrate, the Hertfordshire fire service said.
Fears were growing over the toxic nature of the smoke cloud, estimated to be some 230 kilometres wide.
Meteorologists said the smoke "would almost certainly go over to France when the breeze picks up."
Doctors warning
Doctors warned that the elderly or asthma sufferers could be affected by dust and soot particles, especially near the fire.
Witnesses who saw the first blasts described them in apocalyptic terms, with one likening the scene to "doomsday."
A series of explosions tore sections of walls off buildings in an industrial park, smashed windows and dented doors of homes, ripped tiles from house roofs, burned trees and incinerated at least half a dozen cars over a wide area.
The fire caused widespread traffic disruption including closing down sections of the M1 motorway, a main artery north from London.
The depot, jointly run by French oil company Total and Texaco, part of US oil giant Chevron Corporation, was able to to store up to 150,000 tonnes of fuel, which was distributed to airports in the London area.
Only seven tanks at the depot had remained intact, with firefighters spending the last day cooling them with water to prevent them from exploding.