Astonishing photos showed that part of the defunct coal-fired Didcot A building has collapsed – causing a scene of devastation.
One person has died and other casualties were taken to the John Radcliffe Hospital in Oxford.
Nathan Travis, deputy chief fire officer at Oxford Fire and Rescue Service, said on Wednesday morning that specialist crews were being brought in from the rest of the country to help with the search for the three missing people.
He confirmed there were no explosives on the site but said they had serious concerns for the “integrity” of the building and said the safety of fire crews was his priority.
Mr Travis said they had received some “signs” from search dogs.
Four men are in hospital in a stable and non-life threatening condition while another man is in a serious but not life threatening condition, according to the hospital trust.
Around 50 people were treated at the scene for dust inhalation.
Rodney Rose, deputy leader of Oxfordshire County Council, confirmed one person had died.
He added the incident has being treated as a “collapsed building, not an explosion” but said there was a bang.
Thames Valley Police and a hazardous response team are at the scene.
South Central Ambulance Service described it as a “major incident”.
Six ambulances and two air ambulances were sent to the scene at around 4pm on Tuesday.
Mr Travis said specialist urban search and rescue teams, including sniffer dogs, were at the “challenging” site from as far away as the West Midlands and Hampshire.
He said: “The building is potentially 10 storeys high, half of that building has collapsed, so you have got a rubble pile which is approximately 20 to 30 feet deep at the moment.
“The search will be considerable due to the instability of the site, we expect the search to continue throughout the night and possibly into the coming days.”
The fire service told people living nearby to stay in their homes, adding the dust from the collapse had “covered a considerable area”.
David Cooke, whose company Thames Cryogenics have a building overlooking the power station, said: “Our building shook and as we looked out of the window, the end of the main turbine hall collapsed in a huge pile of dust.
“It totally obscured the towers and must have drifted across the roads and main rail line. What”s left looks a tangled mess.
“The dust was hanging over the area for five to 10 minutes.
“First thought was, it didn”t looked planned, followed by the thought that people are going to have been hurt.”
A huge fire at Didcot power station destroyed four cooling towers in 2014.
Didcot A closed in March 2013 following the passing of the Large Combustion Plant Directive after more than four decades producing power.
The site was able to generate 2,000 megawatts of electricity – enough to meet the needs of two million households.
The station included six cooling towers measuring 114 metres high, which dominated the town”s skyline.
A spokesman for energy firm npower said: “We can confirm that shortly after 4pm this afternoon part of the boiler house at our former Didcot A power station site in Oxfordshire collapsed while an external demolition contractor was working in it.
“Our thoughts are with the families of all those involved in this tragedy.”
Coleman and Company, the firm behind the demolition, tweeted: “We are aware of an incident at Didcot A Power Station. We are working with all stakeholders to establish facts and will keep you updated.”