Explosions tore through a fuel depot north of London before dawn yesterday, creating a huge tower of smoke and flame and seriously injuring four people in what police said appeared to be an accident.

"All indications at this stage are that this was an accident. However clearly we will keep an open mind," Hertfordshire Chief Constable Frank Whiteley told a news conference after Britons, still on edge from July attacks in London, awoke to images of rampaging fire and destruction.

Eye witnesses described a series of massive explosions at the Buncefield oil depot, just after 0600 GMT, spewing flames and a column of black smoke, smashing house windows and causing widespread damage.

More than four hours after the blast, a huge column of black smoke continued to billow several hundred feet into the air.

"It's probably the largest that I have seen ... in terms of actual fire size," the county's chief fire officer, Roy Wilsher, told a news conference.

Wilsher said the fire was under control but could burn for days and there could be more small explosions.

Nearby Luton airport and London's Heathrow said the huge pall of smoke could disrupt some services.

The Buncefield depot supplies petrol and fuel oils for a large part of southeast England. Oil is brought to the depot, near the town of Hemel Hempstead, in an underground pipeline from tankers unloading on Britain's east coast.

Whiteley said there were around 36 casualties, about four of whom suffered serious injuries.

"At the moment it looks as if we got off a lot more lightly than you would expect with an explosion of this size," he said.

The depot is jointly run by oil companies Total and Texaco.

Britain has been on edge for attacks since four suicide bombers blew themselves up and killed 52 commuters in July in London, but the police appeared to rule out any foul play in Sunday's blast.