The most overcrowded prisons in England and Wales are twice as likely to be rated as failing, a new report claims.
According to the Prisons Reform Trust, nearly two thirds of prisons are overfull, with jails holding more than 10,000 inmates than they were built to.
Of the 30 most crowded, 15 are rated as "of serious concern" or "of concern" - the two lowest performance ratings.
The proportion is almost twice as high as the rest of the estate, with 31 of the 117 prisons being similarly rated.
The report, compiled from analysis of Ministry of Justice (MoJ) data, found the three most overcrowded prisons - Leeds, Swansea and Wandsworth - rated as "of concern".
Five of the six jails whose performance is described as being "of serious concern" - Doncaster, Bristol, Isis, Hewell, and Wormwood Scrubs - are overcrowded.
Only one of the eight prisons rated as having "exceptional performance" - Whatton - is overcrowded.
England and Wales' most overcrowded prisons (Oct 2016)
- Leeds: Designed to hold 669 men, it now holds 1,145 - rated as "of concern"
- Swansea: Designed to hold 268 men, it holds 456 - - rated as "of concern"
- Wandsworth: Designed to hold 943, it holds 1,564 - - rated as "of concern"
The Prison Reform Trust said overcrowding "cripples the system's ability to provide a decent and constructive public service".
Peter Dawson, director of the Prison Reform Trust, said: "The bleak state of our prisons is a political failure, shared by all governments of the last two and a half decades.
"Three years of austerity have now brutally exposed the system's inherent vulnerability, and a comprehensive strategy to control the demand for prison, and so to end overcrowding, must form part of this government's response."
A MoJ spokeswoman said it was "clear that our prisons are in need of reform" and it was investing £1.3bn in building new prisons with 10,000 new places.