An incident in which prison officers are said to have lost control of two wings at a jail in Hertfordshire has ended, the Ministry of Justice said.
Disorder broke out at HMP The Mount on Monday and specially trained staff were sent in.
The trouble, said to have spanned two wings which house 227 inmates, erupted on the same day that a report warned the prison has struggled with “severe” staff shortages.
A Ministry of Justice spokesman said the incident had been “resolved”.
The facility has been running on a “restricted regime” in recent weeks and is 47 staff short, prison affairs academic and blogger Alex Cavendish said.
He said inmates have been locked up for 24 hours a day in some cases, and food delivered cold to cell doors.
A woman who said her son is an inmate tweeted: “My son is at The Mount he has been on 24 hour lock down for weeks.
“Quite often he says he’s feeling unwell and I ask if he’s seen a doctor – he said it’s difficult to get to see them.”
Mr Cavendish added that he had warned just days ago that the prison was “on the brink of a meltdown”.
He said: “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.”
He said so-called Tornado squads, equipped to deal with riots, had been sent in.
An assessment from the jail’s Independent Monitoring Board (IMB) said that last summer “all the ingredients were in place for The Mount to suffer disorder such as has been experienced in other prisons – staff shortages, readily available drugs, mounting violence”.
The report covering the 12 months to the end of February said that during the year The Mount has struggled with staff shortages “driven by uncompetitive pay scales”.
It said: “Experienced staff have left and not been fully replaced, so that at the end of February there were 24 vacancies out of a complement of 136 officers, and a high proportion of officers and managers had less than two years’ experience.”
Despite the shortages the IMB said the establishment ended the year as a safe prison where prisoners have a good chance of rehabilitation, with the Governor and staff managing to control violence.
It said the drug problem at the jail was most acute in November when a number of prisoners suffered serious short-term health problems with 70 emergency call-outs in the month, mainly for drug related problems.
The substance known as Spice is a “big concern”, the report said, adding that while drone deliveries have declined, “the supply is still getting in”.
The Category C male prison, which opened in Hemel Hempstead in 1987, has a population of more than 1,000 prisoners.
The facility is described as a “hybrid training and resettlement prison” which caters for a number of prisoners nearing the end of their sentences who are residents of the county and nearby areas.