Prisoners' confidential calls to the suicide prevention charity Samaritans have been recorded and listened to.
Inspectors made the discovery when investigating a similar case of intercepted prisoner calls to MPs. Justice Secretary Chris Grayling disclosed last month that prisoner calls to MPs could be in jeopardy and had asked inspectors to look into it.
A spokesman for the Samaritans told Huffington Post UK, "[We take our] commitment to confidentiality for callers extremely seriously". He added the charity was "very concerned" by the news.
The Samaritans is a globally recognised, confidential phone number yet evidence shows that calls were recorded and played back in a Serco prison.
The private firm Serco, which runs five prisons in the UK, uses a different monitoring system to other facilities that failed to secure the phone calls.
In addition, inspectors found that not all prison staff were aware they should not be listening to these confidential conversations.
Nick Hardwick, Chief Inspector of Prisons, deflected blame from the staff saying, "The system depends on prisoners being aware of their responsibilities to identify confidential numbers and I found that more needed to be done to ensure they understood this responsibility."
Samaritans have said they are now working alongside the National Offender Management Service to ensure the "calls to Samaritans must not be recorded or monitored, are fully understood and adhered to."