Prisoners in Wales were handed an extra 70 years on top of their sentences for breaking rules in the past two years.
The Howard League for Penal Reform called it "draconian" and wants the practice banned.
In Wales' five prisons in 2016 and 2017, 25,555 extra days were handed out to transgressing prisoners.
A Prison Service spokesman said the public would expect any prisoner flouting the rules "to be punished and spend more time in prison".
Additional days can be added for offences including possession of a mobile phone, smuggling drugs or attacking prison staff.
In 2017, inmates in Berwyn, Cardiff, Parc, Prescoed, Swansea and Usk prisons were given an extra 15,214 days on top of their sentences.
This rose from 10,341 in 2016, but HMP Berwyn, in Wrexham, only opened in February 2017 and accounted for 3,455 days in 2017.
Frances Crook, chief executive of the Howard League, said: "The explosion in the use of additional days of imprisonment has been a catastrophe for the prison system.
"Rather than solving problems, it has created new ones - piling more pressure on the prison population and worsening overcrowding, which in turn leads to more drug abuse and violence."
Cardiff prison saw the biggest rise, up from 2,829 to 3,840 days, Swansea's went from 1,219 to 1,511 while Parc Prison in Bridgend handed out 6,358 punishment days in 2017, up from 6,293.
There were no extra days handed out in HMP Usk and none at its sister site, Prescoed, in 2016, rising to 50 in 2017.
The Howard League said its research showed prison culture, rather than size or population, was a better indicator of whether more days were imposed on inmates.
The charity said two people breaking the same rule could receive different punishments, leading to a sense of unfairness.