An investigation is under way today into a massive explosion at a chemical plant.
The blast happened at the Terra Nitrogen (UK) Limited site in an industrial area off Haverton Hill Road, Billingham, Teesside, at 12.15am.
Residents reported hearing the huge blast and seeing a massive ball of flame.
The explosion caused a fire involving mixed gases including hydrogen, nitrogen and a small amount of ammonia and prompted the police to declare a major emergency.
Firefighters and ambulance crews were called to the scene and by 2.45am the blaze had been put out. Two people working at the site were treated for cuts and bruises.
Nearby roads were closed and residents were advised to keep their windows and doors shut as a precaution. The plant, which produces ammonia and was previously owned by ICI, was shut down.
The cause of the blast is not clear but police said an investigation would be carried out by the Health and Safety Executive and the Environment Agency.
Les Gunn, who lives about four miles away from the plant, told BBC News 24 he was woken by an "absolutely deafening" sound.
He said: "There was a huge ball of flame. I am four to five miles away from Billingham and the area of the chemical works but the noise was absolutely deafening.
"I can only describe it probably as like hearing Concorde go overhead."
Joanne Twaits, from Billingham, who said she lived opposite the site in New Road, added: "There was a huge gush of fire and an astonishing thud. It woke my two young children and shook the house as I looked out my window."
Tom Neale in Marske, about 20 miles away, told the BBC: "Many people I know said they saw a flash of yellow light."
Supt Ian Richards of Cleveland Police said the emergency services had the situation under control in a relatively short period of time.
He said: "Fortunately what was initially perceived as a major incident was quickly dealt with.
"The emergency services and petrochemical industry on Teesside regularly exercise to ensure such incidents can be dealt with effectively.
"Their expertise and training to deal with such situations and protect the public is nationally recognised."
He added that the closed roads had been reopened and that there were no "off-site implications" from the incident.
A spokeswoman for Cleveland Fire and Rescue Service added: "A large number of crews were sent to the incident."
Ashraf Malik, UK manufacturing director of Terra Nitrogen (UK) Limited, said: "We would like to apologise to anyone who has been inconvenienced by the need for the precautionary actions and would thank people for their patience and understanding during the incident.
"The company would also thank our employees and the emergency services for their efforts."
A Terra Nitrogen UK spokesman said the firm had launched an investigation into the incident which would involve the regulatory authorities.
He said: "The incident happened on a pipe at the plant and led to a fire involving mixed gases including hydrogen, nitrogen and a small amount of ammonia which was extinguished automatically as the pipe depressurised.
"In line with the plant's emergency procedures, the county police and fire brigade attended the scene working in partnership with the on-site emergency services. The plant was safely shut down."
He added: "At no stage was there any danger to the public."
Terra Nitrogen is the UK's largest manufacturer of nitrogen products used extensively in the chemical industry and agriculture.
The company employs around 400 people in the UK which includes 250 people on Teesside.
Stephen Elliott, chief executive of the Chemical Industries Association told BBC Radio 4's Today programme the blast could have an impact on chemical production.
He said: "Certainly, in terms of the site itself, I think there is bound to be an impact. From what I know it doesn't sound like the incident spread beyond that particular site so I wouldn't expect there to be any restriction on production elsewhere."
He said the firm had a "leading position" in the manufacture of fertilisers.
He added: "It's too early to tell whether we are going to face that sort of shortage because there are obviously other players in the market as well."
He conceded the blast would raise questions about safety.
He added: "We are a high hazard industry so we are heavily regulated and that's right. On top of the regulation there is also a voluntary code that members of our organisation respond to which tries to go beyond the letter of the law.
"The reassurance can only come from learning the lessons here, ongoing commitment to much better safety, health and environmental performance.
"It may not feel like it this morning but the overall track record of the chemical industry in terms of incidents is a good one."