Three men and one woman remain unaccounted for after a fire and two subsequent explosions at a wood flour mill.
Search and rescue teams are to enter the collapsed four-storey building in the village of Bosley, near Macclesfield, Cheshire, as fire crews deal with a number of ongoing fires.
Fire chiefs plan to start the recovery operation today at Wood Treatment Ltd which was described as "a scene of devastation".
An initial report of a fire at the premises at about 9.10am yesterday was followed by two loud explosions - which some witnesses likened to "an earthquake" - and then massive plumes of flames were seen up to 70 metres high.
Smoke continues to emanate from the scene.
Cheshire Police said they were working closely with the families of the four missing but were not releasing names at this stage.
Paul Hancock, chief fire officer for Cheshire Fire and Rescue Service, said: "We are still looking at four people who are unaccounted for at this stage.
"We have some urban search and rescue teams doing an assessment of the scene to determine whether it is safe or not to go into the property and carry out the search, and locate these four unaccounted people.
"We are still in the acute phase of the incident in that the fire crews are still dealing with fires within the building."
He added: "The circumstances down at the scene is one of devastation. When you have two explosions in a building, the building splits open from the inside and as a result of our firefighting tactics over a course of two or three hours it has then collapsed on itself.
"So it is extremely difficult and challenging for those fire crews and those specialist teams to go in to try and locate these potentially missing four people.
"These urban search and rescue teams are teams that went out to Nepal and Japan recently. They have got a big task in front of them. But before we commit these crews we must ensure that they are safe to go in that environment.
"The explosion combined with a number of risks – running fuel fires, LPG cylinders, asbestos ... they are significant hazards."
Assistant Chief Constable of Cheshire Police Guy Hindle said: "Irrespective of the hazards and the danger, we won't give up and we don't give up."
A total of 35 casualties were assessed at the scene at Wood Flour Mills, with four people taken to hospital.
A 29-year-old woman was airlifted to Queen Elizabeth Hospital, Birmingham, after she suffered serious burns and blast injuries to her head, face, arms and chest.
Another two people were taken to a specialist trauma unit at Royal Stoke University Hospital, while a fourth male casualty was taken to Whiston Hospital, Merseyside.
Up to 80 firefighters from Cheshire, Derbyshire, Greater Manchester and Staffordshire fire and rescue services attended and were supported by colleagues from the West Midlands and North West ambulance services, together with Cheshire Police.
Fire chief Mr Hancock said the search operation involving search and rescue team members from Merseyside and Lancashire fire and rescue services would start last night but he could not estimate how long it would take.
He said: "It will take time to go painstakingly through the rubble and debris we will encounter."
He added it was too early to speculate on the cause of the blaze as investigators were likely to be on the scene for several days
Student Charli Alston, 18, who heard the first explosion as she walked her dog, said: "I was walking down my drive and there was just a huge bang. It sounded like a thunder cloud, we didn't know what it was because it was sunny then.
"We looked over and there were massive plumes of flames. Almost as high as the hill next to it.
"I couldn't tell you how high the flames were, but I'd say a good 70 metres into the air. Huge. The sound was really really loud and there was loads of billowing black smoke.
"We phoned 999 straight away, because it was pretty terrifying watching it."
Charlotte Maher, who lives close to the scene of the explosions, said the incident was a "major devastation" for the village.
She told Sky News: "The house completely shook. We thought it was an earthquake.
"Black smoke started coming out and we heard the sirens.
"Because Bosley is such a small village, a lot of people we know work there.
"It's a major devastation for the village.
"We felt two vibrations here five minutes down the road. I can't imagine how the houses down there took it."
Patrick Jones said he was "absolutely gutted" to hear about the explosion after he clocked off his shift at 6am at Wood Treatment Ltd
He said his employers were "brilliant" and added: "You couldn't ask for a better place to work.
"As long as my mates get out, that is all I am bothered about."
Cheshire East Council set up a helpline number for Bosley residents and those affected by the incident.
Council leader Michael Jones said: "Our prayers and hopes are that these missing people will be found alive and well but with the building collapsed we are obviously concerned.
"This has had a major impact on such a very special village. It's a small community and there were 50 jobs there. Fifty workers have lost their jobs and need to find work.
"The community is a good community here but it is shocked, it's traumatised by the events."
David Rutley, Conservative MP for Macclesfield, said: "I have met some of the people who were there when the blast took place. They are clearly in a state of shock.
"It is a huge shock to the community, it's a close-knit community.
"This really hurts. It's been a real shock to the people locally. Wood Treatment has been an important employer here for many many years. So we need to make sure we pull together for the community."
Wood Treatment is part of the Bodens Group, which states that the company has been producing wood flour since 1930 and is the sole manufacturer in the UK, according to the Bodens Group of Companies website.
The helpline number is 0300 1235034.