A suspected leak from a petrol storage tank is thought to have sparked the huge explosions at a fuel depot in Hertfordshire early yesterday, causing the largest fire of its kind seen in peacetime Europe.

Eye-witness accounts suggest that a fault with a three million-litre tank of unleaded petrol at the Buncefield oil depot, near Hemel Hempstead, sparked the explosions. The blast unleashed 300ft flames into the air and sent thick black smoke pouring across the town and beyond.

The explosion, which injured 43 people, two seriously, shattered windows and ripped doors off hinges. The reverberations were felt up to 100 miles away in Europe. The first blast at 6am was understood to have happened at part of the site which supplies Total and Texaco garages, and then spread to tanks containing kerosene for airliners and domestic fuel.

A chemical engineer for a major oil company said that he believed the only way an explosion of this size could have occurred in the cold weather was through a "huge and catastrophic leak of gasoline".

The enormity of the initial blasts prompted initial concerns, played down by police, of a plane crash or a terror attack on the depot, Britain's fifth largest which holds millions of gallons of fuel and supplies aviation fuel for Heathrow and Luton airports. Fires raged for much of the day and explosions continued as around 2,000 people living nearby were evacuated, while police advised others to keep their windows and doors closed because of the thick plume of smoke rising from the flames.

Satellite pictures showed a dense mass dispersing east, west and southwards and by the afternoon a clear and sunny day forecast in London by meteorologists had turned gloomy. Police said it was a "miracle" that no one had died and the low casualty list was because it was Sunday morning with few workers on-site and in the neighbouring industrial estate.

Last night around 100 firefighters were waiting to tackle the blaze but with sporadic explosions throughout the day they may have to wait until this morning, according to Hertfordshire's Chief Fire Officer Roy Wilsher. Emergency supplies of foam concentrate were being delivered from as far away as Humberside. It will be mixed with water to create the millions of tons of foam needed to tackle the blaze.

Mr Wilsher said: "This is possibly the largest incident of its kind in peacetime Europe. The damage a fire of this intensity will cause may, or may not, leave clues for the fire investigation team."

The blaze caused widespread disruption to traffic with the hazy skies causing minor delays at Heathrow and the closure probably until the end of today of the M1 motorway north of the M25 to junction 10. In Hemel Hempstead around 300 evacuees spent the night in a leisure centre while schools, day centres and libraries in the town will be shut today. One fire fighter who was among the first crew ordered to enter the depot for an emergency rescue mission said the incident was one he had dreaded throughout his career. "Before we went in for the search and rescue to the depot I contacted my parents because I didn't know whether I was going to come out.It's been one of these areas we know is at risk."