A prison roof has been set alight by an inmate who stripped off and torched his clothes.
Dozens of prisoners were evacuated while firefighters tackled the blaze at HMP Guys Marsh, near Shaftesbury, in Dorset, in the early hours of Saturday.
It is believed the inmate scaled the roof wearing multiple sets of clothes before setting light to them and triggering the "large fire" at around 8pm on Friday.
The inmate was drunken and protesting against changes to the prison regime, according to the BBC.
Some 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 being treated by paramedics as a precautionary measure and was not immediately arrested.
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.
Superintendent Caroline Naughton said: "This was an isolated incident. An investigation is now under way to establish the exact cause of the fire. These enquiries remain ongoing.
"We would like to reassure members of the community that there is no immediate risk to those living in the surrounding area and that the incident is under control."
A Prison Service spokesman added: "There is no danger to the public and staff are working to bring the incident to a safe conclusion as quickly as possible."