Sixty inmates have reportedly taken control of a wing at HMP Swaleside on the Isle of Sheppey in Kent.
The Prison Officers Association (POA) said there is an "ongoing disturbance" at the category B training prison.
POA chairman Mike Rolfe said the extent of the disturbance is unclear but fires have been lit.
Mr Rolfe said: "There is an ongoing disturbance there (at Swaleside). We are not sure how bad it is yet but we understand 60 prisoners have taken control of 'A' wing there.
"Some fires have been lit, apparently, and basically we know that it's an ongoing incident."
HMP Swaleside has a capacity of around 1,100 inmates serving prison terms of more than four years. It has eight wings comprised of single cells.
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 serious.
A survey found 69% of inmates had felt unsafe at some point - a result which was significantly higher than at similar facilities.
More than half (52%) of prisoners polled said it was easy or very easy to get drugs at the prison, while 45% said the same about alcohol.
HM Inspectorate of Prisons also said the use of force was high, while the segregation unit was described as "filthy".
It is the fourth disturbance at an English prison in less than two months.
Last week, hundreds of inmates rioted for more than 12 hours at Birmingham prison, one of the country's biggest jails.
Prisoners set fires, stole keys to residential areas from a guard and one inmate was seriously injured in a major disturbance on December 16.
On November 6 about 200 inmates went on the rampage at HMP Bedford just days after a national response unit had to be brought in to control prisoners after a six-hour incident at HMP Lewes in East Sussex.
The A wing of the prison houses the Open Academy, a study centre with a computer suite.
At the time of the HMIP report, inspectors said it was used by 50 of the 126 prisoners on the wing, who were enrolled on open and distance learning courses.