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abuse and violence,” Ms Crook said.

“At the same time, rules to incentivise prisoners’ behaviour have been made more punitive, which is also contributing to the poisonous atmosphere behind bars.

“The imposition of additional days should be seen as a sign of a poorly performing prison and included in new measures to monitor safety and order. The rules around incentives and earned privileges must also be revised, as ministers promised.”

Governor or independent adjudicators can stop prison earnings, ban privileges or confine prisoners to their cells – sometimes in a segregated unit.

At Cardiff there were 18 cases of prisoners being confined for 14 days each at in 2016.

A Welsh prison source, who did not want to be named, said: “The Government some years ago, back in the late ’90s changed the rules.

“A governor was able to give up to a maximum of 42 additional days loss of remission. They stopped governors doing that and the only people able to do it then were judges.

“It’s not so good for prison officers because people do not get their just deserts. If a prisoner is violent toward you, for instance, that might just result in a loss of canteen.”

A Ministry of Justice spokesman said: “It is right that prisoners who break the rules, including those on drugs and violence, face sanctions.

“That is why we are working to give governors and those who work in prisons, the tools to reform and rehabilitate offenders more effectively.”