It is "highly unlikely" that three people missing in the Didcot power station collapse are alive, a local fire chief says.
One person has died and five were injured after the "major incident" at the Oxfordshire site on Tuesday.
One of those feared to have died has been named on social media as Michael (Mick) Collings.
A post on Tees Riders MCC Facebook page he was a "good friend and dedicated member" of the club.
Chrissi Hutchinson wrote: "Ride free Mick. You will be missed more than you would ever have realised......so many memories, so many good times."
Shaz Morgan said: "Tragic news RIP Mick. Big lad with a massive heart and infectious laugh."
Oxfordshire Fire and Rescue Service chief fire officer David Etheridge said at a press conference there had been "no signs of life detected" in the wreckage while looking for the missing people.
The families of those missing have visited the site, the emergency services said in a joint statement.
Mr Etheridge said: "We remain committed and determined to return the missing people to their families, and work continues overnight."
Demolition work had been taking place at the decommissioned Didcot A plant.
The collapse, which was initially reported as an explosion, happened at 16:00 GMT on Tuesday.
Mr Etheridge said search and rescue teams had been using thermal imaging cameras, drones with audio sensing equipment and sniffer dogs, but had not detected anything.
The military is using a "mini remote control vehicle to assist with the search for survivors", the emergency services said.
He said the teams had received no response from the demolition workers' radio, which they were treating as "highly significant".
Five people were taken to the John Radcliffe Hospital on Tuesday, with one being discharged last night.
Andrew Stevens of Oxford University Hospitals NHS Foundation Trust said three people were expected to be discharged later and one patient remained in the hospital in a stable condition.
Mr Etheridge said the second half of the building "could collapse at any time" as it has undergone the same process as the first half of the building which collapsed.
He said the operation could take "days if not week" due to the 9m (30ft) high pile of unstable debris.
Vendel Segesdy, who worked at the Didcot site for more than 40 years and helped build the structure which collapsed, said: "I was worried about my safety a week ago.
"It looked like a shell as you could see through the boiler house.
"I thought to myself if there is a lot of weight still left in there this thing could fall like a pack of cards as soon as they start taking pieces out."
During Prime Minister's Questions, David Cameron sent his "sincere condolences to the family and friends of the victim and best wishes to the injured and those still missing".
Ministerial meetings on the matter had already taken place with further meetings taking place later, he said.
An Npower spokeswoman said: "We can confirm that shortly after 16:00 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 that it was "working with all stakeholders to establish facts" and it urged concerned relatives of employees to get in touch.
Mr Etheridge added the families of those missing were "obviously distraught".
Ed Vaizey, the Conservative MP for Didcot, said the man who died and the three people who are still missing were based in the north of England.
He was at the scene on Wednesday and said he was "hoping and praying" for the missing men and their families.
About Didcot A Power Station
Oxfordshire's coal-fired Didcot A Power Station was turned off in 2013, after 43 years in service.
The station included six cooling towers, measuring 375ft (114m) in height, which dominated the skyline of the town.
Hundreds of people gathered to watch when three of the towers were demolished in the early hours of 27 July 2014.
RWE Npower expects complete deconstruction of the site by the end of this year.
A gas-burning power station - known as Didcot B - opened in 1997 on the site and continues to operate.
A major fire was declared at Didcot B in October 2014, with 20 fire crews sent to tackle the blaze, which was caused by an electrical fault.
Assistant Chief Constable Scott Chilton from Thames Valley Police said the service was working closely with the Health and Safety Executive to determine the cause of the collapse.