Some 36 people have been injured - two of them seriously - in a huge explosion at a fuel depot. The blast was so powerful it rocked houses up to 40 miles away.
"Tongues of flames" are soaring hundreds of feet into the sky and an acrid cloud of smoke is stretching for miles from the depot and moving eastwards.
The Army is on standby and people anywhere near the area or close to the plume of smoke are being warned to stay inside and close all windows and doors. Authorities in the region have been handing out breathing masks.
The blast happened at the Buncefield depot near Hemel Hempstead, around 25 miles north of London, just after 6am.
Police have said that although the situation is now under control, they expect more blasts at the site, which stores petrol, kerosene and diesel.
And experts have said that in this type of situation fire crews can only cut off the fuel supply and wait for the fire to burn out.
Police have urged people not to panic-buy petrol. They say contingency plans are in place and there is "nothing to suggest" there will be a supply problem.
Houses near the depot have been evacuated and the M1 has been closed in both directions. The smoke is beginning to obscure main roads, including the M40, reports say.
The cause of the explosion is not yet known but no third party - a plane or a terrorist - was involved.
Map of the area Thousands of viewers are contacting Sky News with reports it shook houses many miles away.
Witnesses have said that buildings have been brought down and the roofs of houses were lifted by the force.
One witness said he was near the site when the blast happened and he saw a building "open up like a doll's house".
Sky Reporter Enda Brady is at the scene. He said there has been a lot of structural damage to surrounding buildings.
"As for residential buildings, it would appear that most people have got away with just blown out windows and cracks," he said.
Brady said he can see a "colossal column of thick black acrid smoke". He added: "People are very, very shocked. Many are walking around in their bed clothes."
Sky News Correspondent Geoff Meade heard the blast at his home.
He immediately called Sky Centre in west London, which was also shook by the explosion, and described "burning tongues" of flames in the sky.
The first blast happened at about 6.03am - it was followed by several smaller explosions.
Sky News producer Anwar Tambe heard the blast from his home in Luton and later became stuck in traffic on the M1. The motorway will remain closed for hours to come.
He said blasts were "going off regularly" and debris has blown onto the motorway.