Footage shows door alarms that did not go off, a hole in a security fence and concerned staff who felt they were not in control.
The reporter spent two months at category C jail HMP Northumberland and at one point was threatened by a prisoner.
It comes as Justice Secretary Liz Truss is to deliver a speech today outlining that prison numbers cannot be cut with “dangerous quick fix” solutions.
In the footage, the BBC Panorama journalist was threatened with a potential weapon when asking if the inmate was going to go back into his “pad”.
A prisoner at Northumberland HMP threatened the undercover journalist with a potential weapon
There is a lot of times when there’s stuff happens and I can’t react to it
Prison staff member
The prisoner can be heard saying: “Move away from me unless you want me to use it on you. Well can you move away from me?”
Inmates were filmed high on drugs and threats of violence were captured.
One man can be heard saying: "Sooner or later someone is going to get killed on one of these wings."
While another says: “You’ve got blood all over your nice carpet.”
A member of staff monitoring CCTV said she could not help in the way she wanted to when trouble broke out.
She says: “There is a lot of times when there’s stuff happens and I can’t react to it the way I want to because there is not the back up there to deal with it.”
Alarms on two doors didn’t work at the prison and wire cutting tools were found near a hole found in an inner security fence.
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HMP Northumberland houses more than 1,300 male inmates and is run by Sodexo Justice Services.
Liz Truss will attack prisoner number opposition in a keynote speech today
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It was privatised in 2014 when the Government aimed to cut the prisons budget by £500million and said the safety of staff and its inmates was its “top priority”.
The Ministry of Justice said it was determined to make its prisons “places of safety and reform.”
Ms Truss will use today's keynote address to attack a "reckless" opposition call to reduce the current number of 85,000 prisoners in England and Wales to the 1990 level of 45,000.
The cabinet member's speech comes ahead the broadcast of the Panorama investigation, which airs tonight.
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