Dozens of inmates have been evacuated from HMP Guys Marsh in Dorset after a fire broke out.
Reports said the fire was started by an inmate who had climbed onto the roof of the building, near Shaftesbury.
Around 60 prisoners needed to rehoused in other areas of the compound after the blaze was started.
Around twenty firefighters attended but police said there were no injuries.
The BBC reported the inmate was believed to be drunk and was protesting against changes introduced at the prison.
After ripping roof tiles from the building the prisoner set his clothes alight.
A Prison Service spokesperson said: “Staff are responding to an ongoing incident involving one prisoner at HMP Guys Marsh.
“There is no danger to the public and staff are working to bring the incident to a safe conclusion as quickly as possible.”