Prisoners have taken control of part of a wing at a prison in Kent with between 60 and 80 inmates involved.
The Prison Officers Association (POA) said there was an “ongoing disturbance” at the category B training prison HMP Swaleside on the Isle of Sheppey.
The Prison Service said it was dealing with “an incident” contained to a landing on the jail’s A Wing.
POA chairman Mike Rolfe said the extent of the disturbance was unclear but fires had been lit.
A Prison Service spokesperson said: “The Prison Service is dealing with an incident involving 60 prisoners at HMP Swaleside.
“The incident is contained to one landing on A Wing. The rest of the prison is secure with all prisoners in locked in cells.”
The BBC’s Home Affairs correspondent Danny Shaw said the disturbance had now been contained but prisoners were refusing to go back into their cells.
He said at least 60 and possibly as many as 80 inmates were involved.
Specially trained guards known as “Tornado” squads have been deployed to Swaleside, the Prison Service said.
The squads were also used to quell a riot last week when inmates rioted for more than 12 hours at Birmingham prison.
Prisoners set fires, stole keys to residential areas from a guard and one inmate was seriously injured.
Up to 240 prisoners were moved following the riot, but the Ministry of Justice has not commented on whether any were shipped to Swaleside.
The prison has a capacity of about 1,100 inmates serving terms of more than four years.
The category B facility gives inmates access to education and training.
An HM Inspectorate of Prisons report in July described the prison as “dangerous” and found levels of violence were “far too high”, with many of the incidents classed as serious.
Latest unrest in England’s prisons
- HMP Birmingham – 16 December – Specialist riot squads were deployed after a riot broke out involving hundreds of inmates and lasting more than 12 hours.
- HMP Bedford – 6 November – A riot saw up to 200 inmates go on the rampage, flooding the jail’s gangways.
- HMP Lewes – 29 October – A national response unit had to be brought in to control the prisoners at the East Sussex jail during a disturbance which lasted for six hours.