Prisoners at a jail struggling with staff shortages have reportedly taken over part of a wing.
A specialist riot squad has been called into action for the second time in as many days at HMP The Mount in Hertfordshire, according to a source.
It is understood prisoners armed with weapons have seized control of part of Nash wing, which houses 250 inmates including many who are serving short sentences or have only three months left to serve.
On Monday, multiple prisoners were involved in a lengthy disturbance across two wings at the jail.
The incident came on the same day as a report warned the prison had lost experienced staff and last summer suffered problems including violence.
Prison affairs academic and blogger Alex Cavendish tweeted: "HMP The Mount: hearing about more trouble this morning. Staff said to have 'lost control' of part of Nash Wing. Awaiting more info."
He then added: "HMP The Mount: report that Tornado Team has been called in. Command suite set up."
During Monday's hours-long disturbance, the so-called Tornado squad, which is equipped to deal with riots, was sent in while police vehicles were pictured at the prison and ambulance crews and fire engines were also seen at a former airfield close to the jail.
The Ministry of Justice said the first incident was resolved shortly after 10pm and no staff or inmates were injured.
It came on the day an assessment from the jail's Independent Monitoring Board said during the year to the end of February The Mount has struggled with staff shortages "driven by uncompetitive pay scales".
The trouble flared days after figures laid bare the scale of the safety crisis that has engulfed jails across England and Wales.
There were 26,643 assaults in the year to March, including a record 7,159 attacks on staff - equivalent to nearly 20 every day.
Campaigners and watchdogs have issued a catalogue of warnings about violence, drug use and overcrowding across the jail estate.
Last month Peter Clarke, the Chief Inspector of Prisons, said staffing levels in many establishments are too low to maintain order and described the conditions some inmates are held in as "squalid, dirty and disgraceful".
A Ministry of Justice spokesman said: "Specially trained prison staff have been deployed to HMP The Mount to resolve an incident involving a number of prisoners.
"The situation at the prison remains contained and there is no risk to the public."
Meanwhile, prison officers have quelled a disturbance at Erlestoke jail in Wiltshire.
A Prison Service spokesman added: "Staff successfully resolved an incident involving a small number of prisoners at HMP Erlestoke on 1 August.
"The offenders responsible will be referred to the police and could spend longer behind bars."
Channel 4 footage from February last year showed then-justice secretary Michael Gove being heckled by prisoners at The Mount as he toured the grounds with political editor Gary Gibbon.
At one point, he shakes hands with an unseen prisoner, who then says "what's your name again?" and, as Mr Gove goes to leave, adds "do a better job than that last one, yeah".