Emergency services said the explosion happened in the Avonmouth industrial area near Bristol.
Four people were killed in an explosion at a sewage treatment plant near the city of Bristol in south-west England.
Chief Inspector Mark Runacres of the Avon and Somerset Police Department said a fifth person was injured in the Avonmouth explosion, but his condition is not life-threatening.
“The police investigation into exactly what happened is at a very early stage and is ongoing,” said Runacres.
He added that the incident had nothing to do with terrorism.
Witness Jawad Burhan said there was a “helicopter looking for missing people” and the police blocked a nearby road leading to the plant.
“I heard the noise, I work next to the building in another warehouse. After 10 minutes I saw the helicopter and the police coming, ”said Burhan.
Prime Minister Boris Johnson said the Avonmouth tragedy victims and their families “are heartbroken”.
“It is deeply sad to learn that four people lost their lives in the waterworks explosion in Avonmouth,” he said on Twitter.
The Avon Fire and Rescue Service said it had been made aware of a large explosion in the industrial area of Avonmouth near Bristol, 195 km west of London, at 11:22 a.m.
The fire department announced that their on-site workers were accompanied by police officers and medical personnel from the South West Ambulance Service in Avon and Somerset.