The Ministry of Justice has said there is an "ongoing incident" at HMP Swaleside in Kent after a disturbance in at least one wing of the prison.
A spokesman said prison officers were "currently managing" the situation in the Category B prison on the Isle of Sheppey which they said only involved a small number of prisoners and said there is not threat to the public.
Several police cars and two Tornado Squad rapid response vans were spotted at the scene on Sunday afternoon but it is not known if they have been deployed so far, Kent Online reported.
The spokesman added: "We are absolutely clear that offenders who behave in this way will be punished and face spending extra time behind bars."
The disturbance comes nearly a year after 60 prisoners took over parts of the prison for 12 hours.
The prison is currently home to 1,100 male prisoners serving sentences of more than four years.
In September this year, the Independent Monitoring Board published a report on the jail saying it had experienced an "unacceptable escalation on instability".
It warned that the prison was becoming increasing dangerous as there was a rising number of assaults by prisoners on both staff and each other as well as te increased use of drugs and weapons.
It said: "Weapons, including blades fashioned in many different ways, are frequently found by staff and are also unfortunately used in assaults.
"This, together with hooch and drugs, is the cause of much of the instability within the prison."
A separate HM Inspectorate of Prisons report in July 2016 described the prison as "dangerous" and said levels of violence were "far too high".
The survey found 69 per cent of inmates had felt unsafe at some point – a proportion significantly higher than at other prisons – and 52 per cent of prisoners said it was easy to get drugs while inside.
The HM Inspectorate of Prisons also said the use of force was high and the segregation unit, where certain prisoners are kept away from the general population, was "filthy".
