A concrete and steel structure at Didcot A collapsed and 5 people are in hospital.
Oxfordshire Fire and Rescue has now confirmed at least one fatality at Didcot Power Station yesterday afternoon (23/02)
More than 50 people were treated at the scene for dust inhalation after part of the Boiler House building came down. Contractors had been preparing it for demolition.
Emergency services have described it as a 'major incident.'
Deputy Chief Fire Officer Nathan Travis updated reporters at the scene just a few hours after the collapse:
He said half of the building was in the process of being stripped out and cleared of hazards, the other half is still unsafe so crews have to be careful in accessing the rubble pile.
Witnesses reported hearing a 'loud explosion' at the site in south Oxfordshire, but Nathan Travis also confirmed there was no explosion.
The fire service says the search for 3 missing people could last for days.
Emergency workers, trained in earthquake recovery, and rescuers with sniffer dogs have been searching through the night for those unaccounted for.
Nathan gave this update just before 9pm (23/02): "There was a partial collapse of a large building, which is approximately 300m long and ten storeys high. The building was due to be demolished in the coming months.
"Sadly, one person has been confirmed to have died during the incident.
"Meanwhile, five people have been taken to the John Radcliffe Hospital in Oxford; two with serious injuries, three with minor injuries.
"There are three people reported missing on site at this time."
The Health and Safety Executive has been informed and will now conduct an investigation, which will be supported by Thames Valley Police.
A 100m cordon was put in place last night.
Nathan added: "The search will be a considerable undertaking due to the instability of the site. We expect the search will continue through the night and possibly into the coming days.
"Dust from the collapse covered a considerable area but I would like to reassure the public there were no hazardous materials within the building, we would advise local residents to remain inside, keeping doors and windows closed.
I would like to confirm that this is not being treated as a terrorist incident.”
A Hazardous Area Response Team, six ambulances and two air ambulances were initially sent to the scene, South Central Ambulance Service said.
This morning, assistant chef fire officer Simon Furlong, said at the scene: “The remainder of the building is very unsafe which is hampering search. This is a very difficult situation with a very unstable structure.
"The safety of emergency service personnel has to remain our priority, while recognising how hard this must be for families waiting for news of loved ones overnight. Our sympathies are with them, and the family of the person who died here yesterday.
"An expert from Cheshire with similar experience is due on site today to advise on the search.
"The police will be taking over control of the site as part of ongoing investigation but we are continuing working with fire services from Bucks, Hants and West Midlands, and this work may continue for several days.”
Thames Valley Police said it was called to the site at 4pm.
"Officers are at Didcot Power Station at an incident with other emergency services. Further details will be released in due course. #Didcot," the force tweeted.
Pictures from the scene show a large part of the a former oil and gas plant on the site - called Didcot A - has collapsed.
David Cooke, whose company Thames Cryogenics overlooks the Didcot 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."
It comes 16 months since a major fire struck a cooling tower at Didcot B, another plant on the site.
Witness Adrian Redhead said it was a "massive explosion... a massive dust cloud came across".
He said: "I looked out of the bedroom window and half the building was missing and rest is hanging on by a thread."
Witness Andy Black, who had been going to a nearby Screwfix store, said: "It looked like the world had ended with dust everywhere. You couldn't see a thing.
"Then I spoke to a guy who said they were not due to blow it until Saturday and then we knew that it was a major problem."
Ed Vaizey, Conservative MP for Didcot and Wantage, told Sky News Tonight: "It is an appalling tragedy. It does appear to have happened as part of the demolition process.
"Obviously we are all devastated to hear about the death of this person working on the site and very, very concerned about the three people who are still missing."
Didcot A opened in 1970 as a coal-fired power station and was later converted so it could also generate power from natural gas.
It ceased generation in March 2013 and hundreds gathered to watch when three of its enormous cooling towers were blown up in July 2014 after dominating the town's skyline for more than four decades.
According to the RWE npower website the total demolition of the site is expected to be complete by the end of this year.
An npower spokeswoman 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."