Roy Wilsher, Hertfordshire's chief fire officer, said there was a possibility Tank 7 could explode so he was withdrawing firefighters in a bid to prevent a "human tragedy on top of an environmental one".

Mr Wilsher explained that Tank 5 had been extinguished during today's operation, but had reignited around 3pm this afternoon.

"This is not much of a problem in itself, but it is nearby Tank 7. We're not sure what's contained," he said. "There is a possibility of extremely volatile fuel."

Mr Wilsher said the fire service was working with the Environment Agency and experts at the depot to determine the contents of the tank. "The worse case scenario is the fire will burn itself out, which will take 72 hours," he said.

Firefighters had managed to extinguish 12 of the 20 burning oil fuel tanks, which was "better than expected", Mr Wilsher said.

However, in addition to the problem with Tank 7, several of the remaining burning tanks had collapsed and were protecting the fuel inside.

But Mr Wilsher stressed that this was just a pause in the operation. "This is a temporary setback and I anticipate we will go back on the offensive as soon as possible," he said.

This morning, 150 firefighters used 250,000 litres of foam to bring the fire - the biggest industrial fire in peacetime Europe - under control.

The blaze sent a plume of toxic black smoke into the sky after a series of explosions early yesterday morning.

Firefighters tackling the flames with foam mixed into 32,000 litres of water a minute described as like putting "a huge foam blanket on a giant chip pan".

The deputy prime minister, John Prescott, said the blaze was thought to be an accident but it would take an investigation from the health and safety executive to make sure.

He said casualties were "remarkably light". In a statement to parliament, he said 43 people had been in hospital. Two of those had serious injuries, but the one person who remained in hospital was described as in a stable condition.

A change in the wind and the cooling of the flames could result in an increase in smoke over Hemel Hempstead. The black plume that has been drifting across southern England is non-toxic but the police are recommending the town's residents should go indoors and shut the windows if it descends.

Smoke and soot can be an irritant to those with respiratory conditions such as bronchitis or asthma. Children from 70 schools in the area were told to stay at home today. Schools in the surrounding boroughs of Berkshire, Buckinghamshire and Bedfordshire were advised to close tomorrow.

The dousing had been due to begin at midnight, but concerns that oil and foam would run off into the water supply meant approval from the Environment Agency did not come until this morning.

Residents and businesses located within the half-mile exclusion zone around Buncefield were invited to a meeting tonight with police, fire service and council officials as well as a representative from Total.

Dacorum borough council said the meeting, to be held at Hemel Hempstead sports centre at 7pm, would update those most severely affected by the situation.

Mr Wilsher said this morning that the fire service was in "uncharted territory".

"This is the largest fire of this kind that we in the UK and Europe have dealt with," he told a briefing earlier today. "We are not even sure how the thermal currents will affect the foam - it may just vaporise it."

The foam forms a film on the surface of fuel that prevents it from reigniting. It is used by all major airports, refineries and other areas where potentially catastrophic fuel fires can occur.

Mr Wilsher said the firefighting plan aimed to keep the water on site. Six high-volume pumps, with the power of 18 conventional fire engines each, drew water from a lake a mile away from the fire to mix with the foam concentrate.

The blasts shattered windows, dented doors and destroyed roofs of houses nearby, forcing 2,000 residents to be evacuated from their homes.

Media reports in France said the smoke cloud from the Buncefield fire was drifting across the country and heading towards Spain.

Frank Whiteley, the Hertfordshire chief constable, warned that because of the risk of Tank 7 exploding, the cordon surrounding the terminal had been widened and the M1 had again been fully closed between junctions six and 10 in both directions.

High temperatures meant firefighters could initially hope only to stop the flames in the 100-acre distribution centre from spreading any further. A "water curtain" kept the flames from the seven unexploded fuel tanks, which each hold more than 3m gallons of fuel.

The Buncefield depot is a major distribution terminal, storing oil and petrol as well as kerosene, and supplies airports across the region including Heathrow and Luton.

The huge explosions initially sparked fears that there had been a terrorist attack or a plane crash. Last week, al-Qaida's deputy leader, Ayman al-Zawahiri, urged militant Islamists to attack oil installations in Muslim countries.

Security sources quickly said there was no indication of "a national security" or criminal connection to the fire.