Fears that cloud may leave toxins in the groundwater

"As yet there is no evidence to suggest this has happened ­ however the situation is being monitored constantly," a spokesman for the Environment Agency said.

And work is also under way to analyse the content of the smoke from yesterday's explosions.

Jane Halpin, Hertfordshire's director of public health, said: "We have got work in hand to sample the smoke to be clear about exactly what it's made up of. We need to know more about this before we can understand what long-term implications there may be, if any, from this exposure." " Dealing with fires is not unusual, it's just the sheer size of it that is different," she added.

The Health Protection Agency is also monitoring the situation and looking at any health implications.

Investigators are now trying to assess the cause of the explosions.

One theory is that there was a catastrophic leak of petrol over a considerable period of time that created a vapour cloud that suddenly ignited.

One of the difficulties in working out what went wrong is the fact that so many different organisations have a stake in the site.

The Buncefield depot is part of a national network of oil storage sites and pipelines that feed fuel from refineries on the coasts to petrol stations and airports around the country.

It is a storage depot for gasoline (petrol), kerosene (aviation fuel) and diesel. Three terminals on the site are estimated to have had a capacity in excess of 150,000 cubic metres when fully laden.

The three terminals are operated by the British Pipeline Agency (BPA), which is jointly owned by Shell and BP; Hertfordshire Oil Storage, which is owned by Total and Texaco; and the third terminal was run by BP. Five pipelines serviced the depot. Four are operated by the BPA, and the fifth is run by TotalFinaElf, a consortium of three oil companies.

A BPA spokesman said that there was an elaborate system of automatic alarms and warnings at Buncefield to alert the operators to any leaks, which produce the greatest risk of accidental fires and explosions.

"A catastrophic explosion like this is a very unusual event, certainly in this country. We're keeping an open mind on the cause," he said.

As soon as the fire alarm sounded after the first explosion, the pipelines supplying fuel to the depot shut off automatically as they were designed to do in an emergency, he added.

The majority of the pipelines are multi-product. This means they carry different grades of refined petroleum products such as gasoline, jet and diesel fuels.

The TotalFinaElf pipeline supplies petrol, diesel and aviation fuel to Buncefield from the Lindsey oil refinery at Killingholme in Lincolnshire.

The BPA pipelines also brought the same multiproducts to Buncefield from refineries at Coryton on the Thames estuary in the south-east and the Stanlow refinery on Merseyside.

The BPA also runs a twin pipeline supplying aviation fuel to Heathrow and Gatwick airports. Road tankers distributed fuel from Buncefield to other airports such as Luton and Stansted as well as to petrol stations around the country.

Buncefield is the fifth-largest oil storage depot in Britain and is part of the UK Oil Pipeline Network, which is owned by a consortium of oil companies and includes more than 400 miles of underground pipelines.

There are no refineries on the Buncefield site, which has been operational since the 1960s, and products are simply stored there for distribution at a later time.

There are other storage sites supplying fuel to Heathrow so there is no imminent danger of a shortage of aviation fuel, the BPA spokesman said.

Other depots are unaffected by the disaster and there is no need for drivers to panic buy, he added. However, long queues were still forming at some petrol stations yesterday.