Wormwood Scrubs prison has been described as "filthy" and unsafe by inspectors.
Nearly half of inmates felt they were in danger during their time at the west London jail, according to a report by Her Majesty's Inspectorate of Prisons.
HMIP said six prisoners had taken their lives since the previous inspection in 2011 - with five deaths in 2013 alone.
The National Offender Management Service (NOMS) said the prison had been through "a difficult change process".
'Much to be done'
The Victorian jail experienced major structural changes in late 2013 that led to a "large tranche of experienced staff" leaving, the report said.
Chief inspector of prisons Nick Hardwick said the findings were "very disappointing".
"There was some recent evidence that important steps had been taken to arrest the decline, but there was still much to be done," he said.
Inspectors said the prison failed to put in place recommendations by the Prisons and Probation Ombudsman to tackle suicide and self-harm.
The jail holds 1,300 prisoners and sees about 2,500 inmates move in and out each month.
At the time of the unannounced inspection in May, 22% of inmates said they felt unsafe.
Danny Shaw, Home Affairs Correspondent
Ranby, Doncaster, Isis...now Wormwood Scrubs. Over the past six weeks there's been a succession of highly-critical prison inspection reports - and the themes have been broadly similar.
Inspectors have identified high levels of violence, concerns about prisoners' safety and welfare and deteriorating conditions.
Is it just a coincidence? Prison governors and officers think not, blaming a combination of staff shortages, changes to prison regimes and an unexpected population surge.
It appears to be a system at full stretch - though ministers insist there's no crisis.
Michael Spurr, chief executive of NOMS, said: "Wormwood Scrubs has been through a difficult change process.
"It has had to adapt to hold young offenders alongside its adult population whilst implementing new structures and routines to provide a decent regime for prisoners at lower cost.
"This has not been an easy transition, however as the chief inspector acknowledges the governor has taken decisive action to address the situation."
According to the report, a number of cells designed for one prisoner held two, many windows were broken with exposed shards, graffiti was widespread and the toilets were filthy.
The report said: "Many staff appeared extremely stretched and some were clearly frustrated that they could not do more; others appeared to have lost focus on prisoners' needs."
Frances Crook, chief executive of charity the Howard League for Penal Reform, said: "People are dying and staff are put in danger as a result.
"Prisons have gone into meltdown in the last year and it is a direct result of government policy.
"I have never seen a public service deteriorate so rapidly and so profoundly."
The Prison Reform Trust criticised the government's "drastic cuts", saying the jail had gone from "getting the basics right to one where standards have deteriorated".