The prisons inspector has called for "urgent" changes to be made at a jail in west London.
Anne Owers said conditions at Wormwood Scrubs in Hammersmith had deteriorated since her last inspection.
Her report said 20% of prisoners given mandatory drugs tests failed them, which had led to gang activity.
The prison had been at the centre of a scandal over abuse of inmates by guards but the inspection found no evidence of an abusive or negative staff culture.
Army camps
But Ms Owers added: "However, disappointingly, it also found that progress had been halted and, indeed, that there had been an appreciable drift in all our key areas - safety, respect, purposeful activity and resettlement.
"This is a direction of travel that needs urgently to be reversed."
The jail holds about 1,200 prisoners including nearly 500 foreign nationals.
Mark Leech, editor of the Prisoner's Handbook, said the 300 new prisoners arriving every month put Wormwood Scrubs under "enormous pressure".
"We have neither the space nor the staff to cope with such numbers and it's inevitable that prisons like Wormwood Scrubs will start to fail in such circumstances," he said.
"The solution is for the government to now open temporary cellular accommodation in disused Army camps."
Phil Wheatley, director general of the National Offender Management Service said: "I am pleased that the inspection has found no evidence of an abusive or negative staff culture at HMP Wormwood Scrubs, something which has been an issue in the past."
He added: "The inspection has refocused the governor and staff at Wormwood Scrubs on driving forward improvements and there has already been success in tackling illegal drug use and progress in implementing the recommendations made by the Inspectorate, specifically in relation to new arrivals and the establishment of a formal personal officer scheme."
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