Tornado squads who are equipped to deal with riots have been sent in to deal with unrest at a prison in Hertfordshire, amid claims two wings are "out of control".
The Ministry of Justice has confirmed that specially trained staff are dealing with an ongoing incident at The Mount.
According to Alex Cavendish, a prison affairs academic and blogger, staff shortages and a "restricted regime" may be to blame for the unrest.
He explained: "The technical term is that the staff have lost control of two wings. What 'lost control' means is that the prisoners are basically rioting, in layman's terms."
The two wings affected in the Category C prison are believed to house a total of 227 prisoners.
A 2015 inspection had concluded that the prison was "reasonably safe" and felt "calm and well ordered", with some "room for improvement".
But Mr Cavendish has claimed he warned the prison was "on the brink of a meltdown" only days ago.
He said he has been told of prisoners locked up for 24 hours a day in some cases, with food delivered cold to cell doors.
Earlier this month, Tornado squads were also sent to tackle unrest at HMP Hewell in Worcestershire.
Official reports had described many challenges at that prison - including serious issues with "safety and respect", high levels of violence and an increase in self-harm among inmates.
The unrest followed warnings from the UK's prison watchdog of "appalling" conditions in jails that have been exacerbated by staffing levels that were "simply too low to keep order".
Peter Clarke, the chief inspector of prisons in England and Wales, said many jails were unable to "run a decent regime" that allowed prisoners to be let out of their cells and access training opportunities or basic facilities.
Assaults on staff increased by 38% last year with a total of 6,844 incidents, and 21 of 29 inspected prisons were judged not good enough on safety.
A Freedom of Information request filed by Sky News found riot-trained Tornado officers were being called to deal with an increasing number of serious incidents in jails - 16 in 2016 compared to three in 2011.
Mr Cavendish said the Tornado squads had been called in to The Mount prison on Monday.
Situated near the village of Bovingdon in Hertfordshire, The Mount is a facility for prisoners who are nearing the end of their sentences and receiving training and resettlement assistance.