The blaze broke out on August 27 and was only fully-extinguished yesterday.

The building at Black Rock Quarry was at the fit-out stage of construction and forms part of a £127m PFI deal to build a new operational base in Bridgewater, two police stations and the tri-force firearms training centre.

An anarchist group calling its self the Angry Foxes Cell claimed on a website that its members were responsible for torching the site.

Detective Superintendent Andy Williams, Head of the Major Crime Investigation Team at Avon and Somerset police said: “Very soon after the fire was discovered, we began a detailed and thorough investigation.

“We brought in a forensic scientist with specific expertise in the field of arson to work closely alongside our own crime scene investigation and forensic teams to get to the root cause of the fire.

“Together, they have concluded that the building was deliberately set alight.

“Our searches for forensic evidence have been carried out in the most meticulous way despite the devastation caused by the fire and our examination of the scene continues still.

“We are under no illusions about the scale of the task we face and are likely to remain at the scene for some time but we will do everything in our power to ensure that whomever is responsible is brought to justice.

“If anyone has any information that might help us then please, call us on 101. “