Lags work through the night to tart up their shabby jail because daytime cleaners are skiving.
Inmates have been painting walls, scrubbing off ingrained dirt and even tackling fly infestations.
They were deployed after cleaners at 1,100-capacity HMP Doncaster in South Yorkshire were found wanting.
A report by HM Chief Inspector of Prisons revealed: “The standards of cleanliness of communal areas varied greatly between wings.
"Some were reasonably clean, others were grubby. We found too many cleaners not working when they should be.
“Many officers did not challenge cleaners. Some showers were in poor condition, with ingrained dirt, poor ventilation, fly infestations, damaged fixtures.
"A cleaning and painting party worked through the night, under close supervision.
“This initiative was beginning to have a positive impact.”
The report also said many lags at the Serco-run jail flouted rules – vaping on landings, wearing inappropriate clothing and playing loud music.
It added: “At times, on some wings, control was tenuous.”
