FIREFIGHTERS resumed their battle last night at what a fire chief called an "apocalyptic" blaze at an oil depot after fears of a fresh explosion receded.

The teams started work again at Buncefield oil depot, near Hemel Hempstead, Hertfordshire, after it was confirmed that a volatile tank contained aviation fuel.

A spokeswoman for Hertfordshire Police 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."

Roy Wilsher, chief fire officer of Hertfordshire Fire Service, said he hoped firefighters would be able to extinguish the blaze by daybreak today.

More than 150 firefighters have been battling to douse the blaze, which began with a series of explosions at 6am on Sunday.

A crew from white watch at Harrogate, North Yorkshire, travelled to Hemel Hempstead on Sunday night.

The four firefighters and an off-icer took with them two high-volume pumps, to supply large amounts of water to be turned into foam.

But earlier last night, Mr Wilsher said senior officers decided to withdraw after becoming concerned that firefighters would be killed.

He said experts were worried that a fuel tank, which had so far remained intact, might explode.

Firefighters needed to be "100 per cent certain" exactly what was in the tank before continuing to battle the blaze.

Mr Wilsher stopped short of saying that firefighters were forced to flee for their lives, but he said the prospect of another massive explosion had concentrated their minds.

Problems arose when a tank unexpectedly ruptured and blew up at about 3pm. Firefighters withdrew, and senior officers decided not to re-enter the site, fearing that the heat generated would cause a nearby tank to explode.

Mr Wilsher said: "For the people working nearby, the rupturing of the tank would have been a very serious situation.

"It would certainly have concentrated their minds."

He said firefighters had set up automatic sprinklers, which were keeping the area cool. These were stopping already extinguished areas from reigniting.

Before being forced to retreat, firefighters had doused 12 of the 22 oil tank fires. The biggest fires were still blazing last night and were due to be dealt with last.

Mr Wilsher said firefighters were using 32,000 litres of water per minute, mixed with foam, to attack the blaze.

Firefighters were trying to cover the blaze with a blanket of foam to deny the fire oxygen and to cool the oil.

Mr Wilsher said: "We are in uncharted territory. It looks devastating.

"It looks like an apocalypse - something people are just not used to."