POLICE have launched an investigation after a prison roof was allegedly set alight by an inmate who 'stripped off and torched his clothes'.

Dozens of prisoners were evacuated from HMP Guys Marsh near Shaftesbury as firefighters tackled the blaze.

It is alleged that the inmate scaled the roof wearing multiple sets of clothes before setting light to them and triggering the "large fire" around 8pm on Friday.

A total of 64 inmates were taken to a secure area in the prison while the fire was brought under control and the scale of the damage ascertained.

A Dorset Police spokesman said: "Officers attended and discovered a large fire on the roof of a building within the prison, which is believed to have been started deliberately."

The prisoner was treated by paramedics as a precautionary measure.

Superintendent Caroline Naughton, of Dorset Police, said: "As it is suspected that the fire was started maliciously an investigation by CID is now underway.

“Those alleged to have been involved in the incident as well as witnesses are being interviewed to progress the investigation, "We would like to reassure members of the community that there was no immediate risk to those living in the surrounding area."

Chief fire officer Ben Ansell said: “Dorset and Wiltshire Fire and Rescue Service remain in attendance following a significant fire at Guys Marsh Prison.

“At the height of the fire there were 21 fire engines and two aerial ladder platforms in use.

“Firefighters have been working overnight in partnership with Dorset Police, the Prison Service, South Western Ambulance Service and partner agencies to bring the fire to a successful conclusion. A joint investigation into the cause of the fire is now underway.”

A Prison Service spokesperson said: "Prison staff worked with the emergency services to successfully resolve an incident involving one prisoner at HMP Guys Marsh on Friday evening.

"We will take the strongest possible action against anyone involved, which could mean extra time behind bars.”

On Sunday, police confirmed a 29-year-old prisoner has been arrested on suspicion of arson with intent to endanger life.

During the prison's last inspection, investigators found it was in "crisis" and managers and staff had "all but lost control".

The unannounced inspection, triggered by "concerning intelligence", found gangs "operated openly" in the facility, which was overcrowded with 543 male inmates in November 2014.

Violence, largely driven by the drug Spice and illicitly brewed alcohol, was "very high" and inmates lived in fear, the HM Inspectorate of Prisons wrote in the 2015 report.

In November, images of inmates eating steaks and takeaways captured on smuggled mobile phones at the prison were condemned as "totally unacceptable" by Downing Street.

The Category C facility holds prisoners serving life sentences as well as those undergoing drug addiction treatment, according to the Ministry of Justice website.