Wormwood Scrubs prison must undergo "urgent" changes to turn around gang, drug and overcrowding problems acording to a report by the prisons inspector.
A quarter of inmates failed drug tests and the problem has led to a surge in the profile of gangs and frequency of bullying inside the Du Cane Road, Shepherd's Bush jail, Anne Owers said.
Forty four per cent of inmates said they feel unsafe and around half the inmates had nothing productive to do to fill the day.
Owers said there had been 'slow but steady" progress in areas such as staff attitudes towards prisoners, in the wake of a scandal over inmate abuse. She also praised the prison's resettledment schemes.
But she warned safety, respect and purposeful activity had "drifted back" to the levels before her inspection in June.
"Like all local prisons Wormwood Scrubs is subject to constant daily pressure. However this inspection identified some gaps and risks in significant areas which required a stronger management grip and greater involvement by residential staff," she added.
The jail holds 1,200 inmates, including 500 foreign nationals and is a category B male prison. 300 new inmates arrive each week.
Phil Wheatley, director general of the National Offender Management Service, said: "It's disappointing that there has been a drift in progress in certain areas, though as the Chief Inspector points out this in the context of continuous operational pressure."