Riot-trained prison staff have been sent to a prison in Hertfordshire after a serious disturbance.
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 have been “lost”.
A Ministry of Justice 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.
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”.
It 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”.
It is understood a command suite to co-ordinate emergency action was set up at 14:00 BST.