A disturbance by prisoners at HMP Birmingham in England has spread to two further wings taking the total to four, sources said.
A specialist riot squad known as the "Tornado Team" has been deployed to quell the trouble which started at the prison - formerly known as Winson Green - in the city centre at around 9am.
All prison officers have also been accounted for at the jail - which is privately run by G4S - with reports of a "badly injured" prisoner and inmates burning and destroying their files.
The Victorian category B jail, which was built in 1849 can hold 1,450 adult remand and sentenced male prisoners - it is also where alleged mass murderer Fred West hanged himself in 1995.
A G4S spokesman said it is continuing to "respond to an ongoing incident" at the prison.
Managing director for G4S custodial and detention services, Jerry Petherick, said: "Our teams withdrew following a disturbance and sealed two wings, which include some administrative offices.
"The disturbance has since spread to two further wings. All staff have been accounted for.
"Additional officers have arrived on site and we have deployed canine units within the prison. West Midlands Police helicopter is also in attendance.
"We are working with colleagues across the service to bring this disturbance to a safe conclusion."
It is understood that in the wake of resources being deployed from across the prison estate management of the incident is now being transferred to the National Offender Management Service (Noms).
It is also understood that around 400 inmates are thought to be caught in the trouble across the four prison wings.
Prison affairs academic and blogger Alex Cavendish told the Press Association an "inside informant" told him the trouble started with lights being broken and inmates controlling fire hoses.
"The officers were then, as they are instructed to do, trying to get as many prisoners locked in their cells as possible to contain it," he said.
"While one of the officers was putting a prisoner in the cell he was threatened with what appeared to be a used syringe."
Mr Cavendish said while this officer was distracted by the threat, "another inmate came up behind, snatched the keys from his belt and snapped the security chain".
He told the Press Association that once prisoners have control of the keys, protocol tells the officers to "withdraw to a place of safety" and said they "abandoned the wing" where the incident started.
The 53-year-old said inmates have also gained access to the offender management unit (OMU), where their paper records are stored, and which have subsequently been burnt.
"I am hearing there is a very badly injured casualty (prisoner), and the prisoners are throwing computers out of the OMU window - destroying records," Mr Cavendish added.
Describing the incident as "probably the most serious riot in a B category prison since Strangeways went up" in 1990, he said: "It is a very serious situation and I think the fact they have now accessed the key offices like the offender management unit - that is where all the offender records are.
"If they destroy all of those records it is going to take months or even years to rebuild the information."
Mr Cavendish said internal connecting security gates have been chained, and that the keys for those are held by the security governor, meaning inmates are "contained" and prevented from "taking control of the whole prison".
A spokesman for the West Midlands Ambulance Service confirmed staff were called to the prison at 12.23pm.
He said it has a hazardous area response team in attendance, together with a senior paramedic officer and a paramedic area support officer, but added that at this time no details are available about any casualties or injuries suffered.