Four people have been killed and one injured after a “large explosion” involving a chemical tank at a waste water treatment works near Bristol.

A major incident was declared at Wessex Water’s Bristol water recycling centre in Kings Weston Lane, Avonmouth, after reports of a blast at 11.22am on Thursday.

Avon and Somerset Police said they were trying to establish the exact cause of the blast.

Chief inspector Mark Runacres told a press conference at the scene that three employees of Wessex Water and one contractor had 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.

“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.”

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.

Luke Gazzard, from Avon Fire and Rescue Service, said the incident had been “very challenging” but that crews had been able to get to it 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.

Witness Jawad Burhan took a photo showing a tank that had exploded.

He told the PA news agency there was a “helicopter looking for missing people” and police had closed a nearby road leading up to the building, which he believed was a waste centre.

“I heard the sound, I’m working beside the building in another warehouse,” he said.

“After 10 minutes I saw the helicopter coming and the police.”

Runacres added that inquiries were under way into what caused the explosion.

He said: “Officers remain at the scene and are likely to be for some time as we work together with other agencies in dealing with this major incident.

“A cordon has been set up and we’d advise members of the public to avoid the area at this time where possible to allow the emergency services to carry out their work.

“Police inquiries into exactly what happened are at a very early stage and are continuing.”

Wessex Water chief executive Colin Skellett said the company were “absolutely devastated” by the incident, and said it would be working with the Health and Safety Executive “to understand what happened and why”.

Sean Nolan, who witnessed the aftermath of the explosion, said he had heard a “boom and echo” but thought it was a vehicle collision.

“I heard what I thought may have been two trucks colliding by the way it shook the ground… it was big,” he said.

“I continued doing what I was doing, but a couple of the lads came out to see if I was alright.

Nolan said there had been “a lot of commotion” in the minutes immediately after, but the incident itself had been “quite short-lived”.

“There were a lot of sirens coming in and some helicopter activity above as well,” he said.

“It (the helicopter) was circling for a good hour or so.

“It was quite short-lived, I’d say about two or three seconds. Sort of a boom and echo and then it just went quiet.

“That was it. There was no smoke, there was no after-effects of it.”

Prime minister Boris Johnson said “our hearts go out” to the victims 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.”