Rescue services said the explosion happened in the industrial area of Avonmouth near Bristol.
Four people have been killed following an explosion at a wastewater treatment plant near the southwest England city of Bristol.
Chief Inspector Mark Runacres of Avon and Somerset Police said a fifth person was injured in the blast at Avonmouth but his condition is not life-threatening.
“Police inquiries into exactly what happened are at a very early stage and are continuing,” Runacres said.
He added the incident was not terrorism-related.
Witness Jawad Burhan said there was a “helicopter looking for missing people” and police closed a nearby road leading up to the plant.
“I heard the sound, I’m working beside the building in another warehouse. After 10 minutes I saw the helicopter coming and the police,” Burhan said.
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 waterworks explosion in Avonmouth,” he said on Twitter.
The Avon Fire and Rescue Service said it was alerted at 11:22am to a large explosion in the industrial area of Avonmouth located near Bristol, 195km (120 miles) west of London.
The fire service said its personnel were joined at the scene by Avon and Somerset Police officers and medics from the South West Ambulance Service.