Letters between inmates and their lawyers have been opened by staff in breach of prison rules, a report has found.
The Prisons and Probation Ombudsman (PPO) carried out 32 investigations into complaints about legal and confidential correspondence being unsealed between April last year and June, ruling in favour of the prisoner in 16 of the probes.
However, the watchdog said most cases were down to "human error" and there was "little evidence of deliberate or sinister tampering" or of repeated failures at an individual facility.
Under Prison Rule 39, an inmate's correspondence with courts and their legal adviser can only be opened, stopped or read in specific circumstances, such as where officials suspect letters contain illicit items.
The PPO Nigel Newcomen's report said that in general, investigations found "one-off and occasional errors", although there was a small number in which staff training or processes had not been sufficient to prevent repeated mistakes.
It said: "However, to say that the evidence pointed to human error rather than deliberate interference is not to minimise the seriousness of the issue.
"Prison Service policy is clear that even accidental breaches of Rule 39 open the possibility of legal challenge."
Mr Newcomen called for prisons to ensure correspondence logs are sufficiently detailed, errors are recorded and staff fully understand guidance on observing the rules.
He said: "Prisoners need to be able to have confidence that their confidential communications will be respected. Even infrequent errors will undermine this."
A Prison Service spokesman said: "These were isolated cases. The recommendations made by the Prison and Probation Ombudsman have been accepted and the necessary action has been taken at the prisons concerned.
"This bulletin is being shared with all prison staff to reinforce the correct process that needs to be followed when handling prisoners' legal and confidential mail."