Inferno.

Byline: By James Horden

Firefighters today began tackling one of the biggest fires in Britain since the Second World War.

The operation to create a blanket of foam to put out the fire at the Buncefield oil depot near Hemel Hempstead in Hertfordshire should have started last night but was delayed after fears that water supplies could be affected.

But Hertfordshire police said an attempt was being made today to douse the fire by using 250,000 litres of foam, mixed with 25,000 litres of water per minute.

"A specialist crane has been brought in to assist the operation," a spokesman added.

Firefighters had spent yesterday containing the blaze with a curtain of water between the raging flames and the remaining unexploded fuel tanks.

The water will be pumped from the Grand Union Canal, two miles away, using high-pressure hoses.

Seven out of more than 20 tanks remained intact and each is said to hold more than three million gallons of fuel.

Meanwhile investigators were waiting for emergency services to make the Buncefield site safe before trying to find out the cause of the fire.

An investigation and demolition of dangerous structures at the depot could take months and the cause of the fire may be difficult to determine, safety experts said.

One expressed surprise that the petrol, aviation fuel and oil products stored at the depot were capable of causing such a massive explosion.

Dr Ivan Vince, a safety, health and environmental specialist, said they should not have been capable of generating such a massive explosion.

"I am sure there is reasonable explanation," he added. "For example, if liquefied petroleum gas was stored there and a vessel exploded, that could cause an explosion of this magnitude."

Dr Vince said he didn't believe there were any serious dangers to the public as the smoke plume was buoyant and was being lifted high above the ground. It would mix with air and be greatly diluted before any of it came back down to the ground.

However, doctors have warned that asthma sufferers and the elderly may be more susceptible to the effects of the plume of black smoke.

And residents close to the site have been warned to stay inside their homes with their doors and windows closed.

Prof Warren Lenney, of the British Lung Foundation, said breathing in the polluted air could cause swelling to the lungs and even affect the brain.

"The trouble is there are all sorts of different chemicals in the smoke," he said. "Petroleum products are known to produce a whole series of nasty acidic chemicals, as well as carbon monoxide.

"It is potentially possible that all of these will be inhaled. Obviously, the nearer you are to where the fire is the worse it can be."

While staff at Hemel Hempstead General Hospital said that the initial casualty influx was now over, extra beds and intensive care facilities were laid aside for anyone suffering complications as a result of the smoke.

Howard Borkett-Jones, medical director of the hospital, said staff had been told the smoke was of low toxicity.

However, they were prepared for some respiratory problems, he said: "Those could include feelings of tightness in the chest or coughing."

Doctors were being advised by the Poisons and Toxicology Unit in London on how to deal with chemical incidents.