One person has been killed, three people are missing and five are in hospital after a building collapsed at Didcot Power Station.
Emergency services declared a major incident after the iron and steel structure, which was being prepared for demolition, came down at the south Oxfordshire site around 4pm.
A rescue operation is expected to continue into the night and the coming days, deputy chief fire officer Nathan Travis has said.
He described the operation as “very challenging” and said it involved specialist urban search-and-rescue teams, including sniffer dogs.
“The building was potentially 10 storeys high – half of that building has collapsed so you”ve got a rubble pile which is approximately 20 to 30-feet deep,” he said.
“The search will be considerable due to the instability of the site.”
When asked what the chances were of finding the missing alive, he said: “At the moment I can”t give you any details on that but it is a substantial collapse of a building.”
Witnesses described hearing what sounded like an explosion before a huge cloud of smoke spewed above the county.
But Mr Travis said: “Because it was being prepared for demolition, explosives would have been on the site but they were all cleared away so it was a collapse of the building [not an explosion].”
The five injured – all men – are being treated at the John Radcliffe Hospital in Oxford. One is in a serious but non life-threatening condition, the others are stable.
The collapse happened at Didcot A, a former oil and gas plant which shut down three years ago.
Workers had been getting the building ready for its demolition, which was expected to happen in the coming months.
A GMB union official told the Press Association: “We understand that workers were preparing two boilers for demolition in the coming weeks. This led to the collapse of a building.”
Specialist police search units, six ambulances, two air ambulance helicopters, fire engines and a hazardous area response team were at the site.
More than 50 people were treated at the scene for dust inhalation caused by the collapse.
People living nearby were advised by the fire service to stay indoors as the dust had covered “a considerable area”.
There had been initial fears about hazardous material at the site, but it was later confirmed that all hazardous material had been removed some time ago.
Didcot B, a natural gas plant which provides power to the National Grid, is still functioning and has not been affected by the collapse.
Didcot A ceased operation in March 2013 after more than 40 years or generating power for the area.
It was originally a coal-fired station, but was later converted so it could also generate power from natural gas.
A huge fire ripped through Didcot B in October 2014. No one was injured, but the blaze put half of the gas-fuelled plant – which supplies a million homes – out of action.