Riot-trained prison staff have been deployed at a prison in Hertfordshire following reports of a serious disturbance.
A spokeswoman for the Ministry of Justice said: "Specially trained staff are working to resolve an incident involving a number of prisoners at HMP The Mount.
"The prison is completely secure and there is no risk to the public."
The riot unit is believed to have entered the prison at around 2pm on Monday.
The prison, which opened in 1987, has a population of more than 1,000 male prisoners and is described as a "hybrid training and resettlement prison" which typically caters to prisoners who are reaching the end of their sentence.
An inspection in 2015 found the prison was performing well, with inspectors noting it was "reasonably safe and felt calm and well ordered", but chief inspector of prisons Nick Hardwick added that there was "room for improvement".
Prisons expert and blogger Alex Cavendish said he had warned of a "disaster waiting to happen" at the jail, where he said there was a shortage of nearly 50 staff.
He tweeted: "HMP The Mount: inside sources state staff have lost control of at least one wing, possibly two. This is attributed to staff shortages.
"HMP The Mount has been running on very restricted regime for weeks. Food served cold at cell doors. Hearing nearly 50 staff short."
He said so-called Tornado squads, equipped to deal with riots, had been sent in.
He wrote: "Tornado Teams called in to take back control of two wings. This has been a disaster waiting to happen as I warned last week."
He also warned that he had been told by an "inside source" that the problems may have been exacerbated by the fact that many young inexperienced prison officers had been deployed.
"[The] prison has been on the brink for weeks," he added.
A man claiming to be a prisoner in another part of the prison contacted The Independent to say they were rioting because they "have not had a shower since Wednesday".
The unnamed prisoner, who said he was calling from his cell, said treatment of prisoners was "in breach of human rights".
"They been doing all sorts of things like banging up for three days or four days and not letting us out. They will let out like selected individuals," he said.
"We've been going three, four days with no shower, no exercise, no nothing."
He said if conditions were not so poor, there would be no rioting.
"You can't keep someone in their cell for days for no reason and not have them try to get out," he added.
The man said he had been in the prison since the start of year and that conditions had deteriorated during that time.
The riot in Hertfordshire comes just a week after the Tornado squads were deployed to HMP Hewell near Redditch in Worcestershire following a disturbance at the Category B prison.
Additional reporting by PA
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