An undercover BBC reporter spent two months at HMP Northumberland, which houses up to 1,348 male inmates, for Panorama.
According to BBC news, the reporter discovered “widespread drug use, a lack of control, door alarms that did not go off ... and a hole in an internal security fence.”
The Ministry of Justice has said it will investigate the "extremely serious allegations" at the Acklington jail.
BBC Panorama spent two months inside one of the country's biggest prisons. During the secret filming, the reporter also recorded scenes including:
• Prisoners incapacitated by drugs
• Officers sometimes left on their own to manage large groups of inmates
• Inmates threatening staff
The Panorama investigation came just days after the Ministry of Justice announced the replacement of the National Offender Management Service with a new prison and probation service aimed at cutting crime and reforming offenders.
Privatised in 2014, HMP Northumberland is run by Sodexo Justice Services. To win the contract, Sodexo pledged to save the taxpayer £130m over 15 years. Two hundred jobs, including 96 prison officer posts, were cut.
HMP Northumberland is a training prison that is said to offer a range of education and training programmes to prepare inmates for release. The Panorama reporter witnessed some inmates colouring in pictures of the children's cartoon character Peppa Pig in an "employability skills" class provided by outside contractor, Novus.
It told Panorama it had investigated these concerns and sent a report to the government.
The president of the Prison Governors Association, Andrea Albutt, told the BBC, "The situation is that there are so few prison officers at the moment; their confidence has been affected and we have a more violent prison population."
The Ministry of Justice told the BBC: "The justice secretary has been clear that levels of violence and self harm in our prisons are too high, which is why we are investing an extra £100m annually to boost the front line by 2,500 officers.
"These are longstanding issues, which will not be resolved in weeks or months but we are determined to make our prisons places of safety and reform."
A spokesman for Sodexo told the BBC: "We are proud of those staff at HMP Northumberland, who do a professional job in such difficult circumstances. Security and the safety of our prisoners and staff are our top priority, which is why we have made significant investments in these two areas over and above the contract requirements."
As part of the investigation, Panorama analysed what prisoners had been saying about safety in prisons across England and Wales.
