HMP The Mount: Second day of trouble at prison
Riot-trained staff have been sent to HMP The Mount in Hertfordshire for the second day running.
BBC home affairs correspondent Danny Shaw said armed prisoners took over Nash Wing and smashed windows, according to well-placed sources.
The Ministry of Justice said order was restored shortly before 17:00 BST.
About 30 inmates at Erlestoke prison in Wiltshire also became violent and four people were taken to hospital, the Prison Officers Association said.
At Erlestoke a prison officer suffered a broken jaw, it added.
The acting chairman of the association, Mark Fairhurst, told the BBC that more than 80 inmates were unlocked on a wing at the jail at the time - with "a nucleus of about 30 prisoners being involved in the violence".
The MoJ has confirmed that some prison officers were taken to hospital but would not confirm the number or the nature of their injuries.
'No risk to public'
Following the end of the problems at The Mount, a Prison Service spokesperson said: "Specially trained prison staff have successfully resolved an incident at HMP The Mount on 1 August. There were no injuries to staff or prisoners.
"We do not tolerate violence in our prisons, and are clear that those responsible will be referred to the police and could spend longer behind bars."
Prison facts: HMP The Mount
The Mount Prison opened in 1987 as a young offenders institution.
It was designed as a category C training prison built on the site of a former RAF station on the outskirts of Bovingdon village, Hertfordshire.
A report in 2016 by the Independent Monitoring Board (IMB) found "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 prison now has a population of more than 1,000 prisoners.
The Nash wing is fairly new - it was completed in 2015 - with 94 double cells and 62 single cells.
On Monday at least 50 cells are thought to have been damaged when violence broke out in two wings at The Mount. No-one was injured on that occasion.
A Hertfordshire Constabulary spokeswoman said: "Following [Monday's] incident we are working with the Ministry of Justice and are currently reviewing what, if any offences, have occurred."
The Mount, near Hemel Hempstead, opened in 1987 and is classed as a category C male prison. Nash wing is believed to hold between 200 and 250 inmates serving short sentences or with only three months left to serve of their sentence.
HMP Erlestoke is also a category C men's prison, which the Prison Reform Trust describes as one where "prison staff think [inmates] will not escape", while acknowledging they "cannot be trusted in an open prison".
Analysis: Danny Shaw, BBC home affairs correspondent
Incidents take place in prisons all the time; it's usually problems with one or two individuals, and most go unreported.
When an incident like this take place - when tornado (specialist riot) teams are involved - it takes it up to a new level.
When you have prisoners rioting or barricading themselves in, with staff having to withdraw, that is very serious.
Problems at the Mount have been brewing for some time.
The independent inspectors that go into prisons were saying they were 25 staff short out of 136 - that is a significant shortfall.
Last week the Independent Monitoring Board (IMB) published its annual report into Erlestoke, and said that the smuggling of tobacco, which has been banned by the prison, had contributed to bullying.
Before the violence broke out on Monday, the IMB also published its annual review into conditions at The Mount and said it had "struggled" with staff shortages.
There were 24 vacancies out of a total of 136 officers in February, it added.
It also claimed violence "grew considerably" throughout the year and that drugs were readily available, in particular the synthetic cannabis substitute spice.
The report said concerns raised last year had not been addressed by the MoJ.
Prison facts: HMP Erlestoke
HMP Erlestoke is a Category C establishment holding adult male sentenced offenders.
A 2017 IMB report on the prison found that lack of experienced officers was a concern.
- Tobacco smuggling had become an issue since the prison became smoke free on 1 April 2016
- Abuse of the drug Spice was an increasing concern
- A death from Spice was "only a matter of time"
The prison has 388 prisoners with 115 of them serving a life sentence.
About 60% of inmates are aged between 21 and 39.