Emergency services were called after reports of an explosion involving a chemical tank at Wessex Water’s Bristol water recycling centre in Avonmouth at around 11.20am on Thursday.
Chief Inspector Mark Runacres, from Avon and Somerset Police, told a press conference at the scene, off Kings Weston Lane, that three Wessex Water employees and one contractor died in the blast.
A fifth person was also wounded, but their injuries are not thought to be life threatening.
Mr Runacres said authorities would not be speculating as to the cause of the explosion – but confirmed it was not being treated as a terror-related incident.
He told the briefing: “Specially-trained officers have this afternoon made contact with each of the families of those individuals and informed them of the sad news.
“This is a tragic incident and our thoughts and sympathies go out to them.”
Steve Preddy, Unite regional secretary for the Southwest, said the union had three members working at the Wessex Water site in Avonmouth, all of whom were uninjured, although in a state of shock.
Wessex Water chief executive Colin Skellett said: “We are absolutely devastated that the tragic incident at our site earlier today has resulted in four fatalities.
“Our thoughts are with the families, friends and colleagues of those involved.
“We are working with the Health and Safety Executive to understand what happened and why.”
The prime minister, Boris Johnson, said “our hearts go out” to the victims of the Avonmouth tragedy and their families.
“Deeply saddened to learn that four people have lost their lives in the water works explosion in Avonmouth,” he wrote on Twitter.
“Our hearts go out to the victims and their families.
“Thank you to the emergency services who attended the scene.”
Witness Jawad Burhan shared a photo on Twitter on Thursday afternoon showing a tank that had exploded.
He said there was a “helicopter looking for missing people” and police had closed a nearby road leading up to the building.
“I heard the sound; I’m working beside the building in another warehouse. After 10 minutes I saw the helicopter coming and the police,” he told PA news agency.
Kieran Jenkins, another witness, described how the warehouse he was in at the time of the explosion shook with the force of the blast.
He told ITV West Country: “The whole warehouse was shaking and me and my colleague were just stood there in shock and the next thing you know we hear people running over, screaming.”
Luke Gazzard, from Avon Fire and Rescue Service, said the incident had been “very challenging” but that crews had been able to attend the scene quickly.
“Luckily our local fire station is not too far from here so we had crews in attendance very, very quickly and they were able to gain an element of situational awareness,” he said.
“Our initial crews attended within five minutes and in total we had six fire engines, a turntable ladder, urban search and rescue teams and two search and rescue dogs on scene.
“Our thoughts are with the families, friends and colleagues of all those affected in today's incident. It is a terribly sad incident.”
Mr Gazzard was unable to confirm where on the site the casualties were found, adding that he was currently “unaware of the position” of the additional casualty mentioned by police.
Labour leader Sir Keir Starmer gave his condolences on Twitter, writing: “My thoughts are with all those who tragically lost their lives today in Avonmouth. My heart goes out to their friends and family.”
Darren Jones, MP for Bristol North West, who earlier said he would be calling on the Environment Agency and Health Safety Executive “to ensure lessons are being learnt for the future” said he and his family would be keeping those affected in their thoughts and prayers.
He added in a tweet: “Can I thank the emergency services – including Avon and Somerset Police, the South West Ambulance Service and Avon Fire and Rescue – for their responsiveness and support.
“I’m pleased that the situation has been contained and that there is no further risk to local people.
“There will be a full investigation taking place but, for now, we pay our respects to those who have suffered and lost their lives today.”
Additional reporting by PA
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