Riot-trained prison staff have been sent to a jail in Hertfordshire amid reports of violence on two wings.
Sources have told the BBC’s home affairs correspondent Danny Shaw that one wing of Mount Prison, in Bovingdon village near Hemel Hempstead, and half of another wing had been “lost”.
A Ministry of Justice (MOJ) spokeswoman said: “The prison is completely secure and there is no risk to the public.”
The Mount opened in 1987 and is classed as a category C male prison.
Riot trained staff arrived at the jail at about 18:30 BST equipped with shields and batons while fire, police and ambulance crews were on standby outside.
The MOJ said officers were dealing with an “incident involving a number of prisoners”.
The BBC understands that the wings involved are H and L, which house 110 and 117 people, and a command suite to co-ordinate emergency action was set up at 14:00.
The Prison Reform Trust calls this type of institution one where “prison staff think [inmates] will not escape,” while acknowledging they “cannot be trusted in an open prison”.
Prison affairs academic and blogger Alex Cavendish had tweeted on Saturday: “Staff shortages at HMP The Mount (Herts) are so severe that this is the 3rd weekend of total lockdown. Meals given at cell door. Trouble brewing.”
On Monday, the Independent Monitoring Board published its annual review into conditions at Mount Prison 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 that violence “grew considerably” throughout the year and that drugs were readily available, in particular the synthetic cannabis substitute Spice.
The report says concerns raised last year had not been addressed by the MOJ.
The Mount is built on a former RAF station site and has more than 1,000 prisoners, according to the Ministry of Justice.
It is described as a “hybrid training and resettlement prison” for prisoners in the final six months of their sentences.
A 2015 inspection of the prison found The Mount was “reasonably safe and felt calm and well ordered”, but chief inspector of prisons Nick Hardwick added that there was “room for improvement”.
In March 2016 an inmate at The Mount stabbed a fellow prisoner with a shard of glass from the microwave.