The number of inmates in Ranby Prison who self-harm reached a record high in 2018, new figures show.
It mirrors a violent trend in jails across England and Wales, where deaths, assaults and self-harm incidents were all at their highest levels last year.
The prisons minister, Rory Stewart, admitted the figures were “unaccaptably high”, and warned that the effort required to improve should not be underestimated.
In Ranby, which has a capacity for 1,038 male prisoners, 409 self-harm cases were recorded last year, with 21 inmates referred to hospital.
This was up from 396 in 2017 and was the highest figure since incidents were first logged in 2004.
It also represented a fourfold increase since 2013 when only 89 incidents of self-harm were recorded.
The latest data also revealed a threefold increase, to 472, in assaults at Ranby in 2018. Of these, 40 were defined as serious, including sexual assaults and victims who required hospital in-patient treatment.
Mr Stewart said: “Violence and self-harm in prisons remain unacceptably high, but there are early signs across the UK that we are making progress.
“Our dedicated prison staff deserve enormous credit and I want to thank them for all their tireless work. There is still much to do, however, to return to long-term stability.”
Frances Crook, chief executive of the Howard League for Penal Reform, said: “The latest figures reveal the sheer scale of the challenge to reform a prison system that has been over-burdened and under-resourced for years.
“Change is happening, but it is like trying to turn around an oil tanker.
“I believe the will is there among government ministers to make prisons safe, but lives can only be saved, and violence reduced if the nettle of reform is grasped.”