Firefighters trying to tackle a huge fire at the Buncefield fuel depot in Hertfordshire were forced to withdraw from the compound this evening over fears that volatile fuel in one of the tanks could catch fire and explode.

Because of the risk, police again closed the M1 around Hemel Hempstead, disrupting traffic along the main motorway linking London with the Midlands and North of England.

Roy Wilsher, the county's chief fire officer, said that the firefighting team, which numbers over 150 people from 17 different fire services, had successfully extinguished fires in 12 of 20 burning fuel tanks in Britain's fifth largest fuel depot.

But two problems disrupted the smooth running of the operation. First, one of the tanks where the fire had already been extinguished ruptured and caught alight. Then officers decided that another tank, known as No 7, could contain "extremely volatile fuel" that could cause a fresh explosion if it ignites.

"Until we're absolutely sure what's in that tank, we have withdrawn our firefighters," Mr Wilsher said. He described it as a "temporary setback" that could delay operations for hours, and said that unmanned self-standing pumps were covering the extinguished tanks with a curtain of water to prevent them from reigniting.

The fire erupted shortly before dawn yesterday with a series of explosions that could be heard at least 40 miles away. Windows were blown out in houses in nearby Hemel Hempstead and a vast barrel of smoke rose into the sky and spread slowly across the Home Counties.

Remarkably, however, the blasts caused only one serious injury. A worker in the depot was taken into intensive care at Watford General Hospital last night with lung injuries although his condition was today reported to be stable. Forty-one people were treated for minior injuries and one man was kept in hospital overnight for observation, but released this morning.

Had the disaster happened on a working day, hundreds of people would have been in the depot and the surrounding industrial estate.

In Parliament today, John Prescott, the Deputy Prime Minister, lavished praise on the fire services for their professionalism and courage. But his shadow on the Tory benches, Caroline Spelman, urged him to create a public inquiry into the circumstances of the accident, pointing out that the British Pipeline Agency Ltd was prosecuted for safety breaches at the site four years ago.

The firefighters have been using 32,000 litres of water a minute to spray a blanket of foam over the tanks to contain the fire and gradually extinguish it. The fires tackled initially were described as "rim fires" licking around the outside of outlying tanks that had not fully caught fire.

At least 250,000 litres of foam were to be used, but Mr Wilsher said earlier today that it was a technique that had never been tried on this scale. "We are in uncharted terrority," he said. "This is the largest fire of this kind that we in the UK and Europe have dealt with. We are not even sure how the thermal currents will affect the foam, it may just vaporise it."

The explosions came only four days after the release of an al-Qaeda videotape posted on the internet that called for attacks on facilities carrying oil that it said had been "stolen" from Muslims in the Middle East.

Frank Whiteley, Chief Constable of Hertfordshire Police, said that there was nothing to suggest that it had been started deliberately. "While we continue to keep an open mind, there is nothing to indicate anything other than an accident," he said.

Police said that one man was arrested at the site last night because he could not satisfactorily explain his presence in the security cordon to officers. He was still being questioned today.

The M10 motorway has remained closed in the Hemel Hempstead area but the M1 was reopoened for most of the day.

The Hertfordshire Fire and Rescue Service had to wait for clearance from the Environment Authority before beginning the operation to put out the blaze because of fears that the run-off from the chemical foam - known as AFFF - could pollute the local water table. So far they have managed to contain the run-off in an empty reservoir at the Buncefield depot.

Weather conditions have favoured the firefighters. A dry and largely windless day saw the think plume of black smoke rise high into sky yesterday before drifting to the south and east. Today the wind picked up and the plume of smoke was pushed southwest towards Buckinghamshire, Bedfordshire and Hampshire.

As the smoke cools, and with some light rain forecast today, the soot from the fire will start to settle, but exactly where and how quickly is still unknown.

The top public health official in the district, Dr Jane Halpin, today advised residents of Hemel Hempstead and other affected areas to stay indoors, keep their windows closed and listen to local radio stations for further advice.

But Dr Halpin said that the smoke did not yet appear to be causing any significant public health problems and only three people had visited Hemel Hempstead hospital with respiratory problems, all of whom had been quickly released.

She said: "Now, as the fire is brought under control and the temperature falls, smoke and soot will fall to earth. These can be irritants and the people most at risk are those with respiratory problems. Of particular concern are the very old and they very young."

An estimated £200 million worth of fuel has gone up in flames at the depot and other businesses with premises in the surrounding industrial park were today counting the cost of the disaster. Among the companies were affected are Scottish & Newcastle, Britain's biggest brewer, DSG International, formerly known as Dixons, and Northgate Information Solutions, a specialist software firm.

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