Four people have died following an explosion at a waste water treatment works near Bristol, Chief Inspector Mark Runacres of Avon and Somerset Police said.

Chief Inspector Mark Runacres, from Avon and Somerset Police, told a press conference at the scene that three employees of Wessex Water and one contractor died in the incident.

“Emergency services were called at approximately 11.20am to reports of a large explosion involving one of the chemical tanks at the site off Kings Weston Lane,” he said.

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“The fire service led the rescue operation but sadly, despite the best efforts of all those involved, we can confirm there have been four fatalities. This includes three employees of Wessex Water and one contractor.

“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.”

Mr Runacres said authorities would not be speculating into the cause of the explosion – but he added it was not being treated as a terror-related incident.

“A fifth person was also injured but their injuries are not believed to be life-threatening,” he told the press conference at the scene.

“A thorough investigation will be carried out into what happened today, involving a number of agencies and the Health and Safety executive in due course, and we will not be speculating on the cause of the explosion while such work is being undertaken.

“I can confirm though that this is not being treated as a terror-related incident.”

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 said on Twitter.

“Our hearts go out to the victims and their families.

“Thank you to the emergency services who attended the scene.”

Darren Jones, MP for Bristol North West, tweeted: “My family and I are keeping those affected in our thoughts and prayers, following the tragic consequences of the explosion in Avonmouth today.

“Can I thank the emergency services – including Avon and Somerset Police, the South West Ambulance Service and Avon Fire & 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.”

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.

“During the incident we located four casualties who unfortunately were confirmed as deceased.

“Our thoughts are with the families, friends and colleagues of all those affected in today’s incident.

“It is a terribly sad incident.”