A prison where 20 men have taken their lives since 2011 has a "staggering" death rate and is still failing vulnerable inmates, a report has said.
Inspectors at HMP Woodhill in Milton Keynes also said there were "chronic and substantial" staff shortages.
The chief inspector of prisons warned it risked undermining the work that had been done to improve the care of those at risk of self harm.
The HM Inspectorate of Prisons said the prison was "still not safe enough".
The report said Woodhill had "deteriorated significantly" in this area since the previous inspection in 2015.
HMP Woodhill holds just over 600 men, mainly remand prisoners and those serving short sentences, alongside a small number of category A high-security prisoners.
In the five years to 2017, it was the prison with the highest suicide rate in England and Wales.
The report said: "At the time we inspected, eight prisoners had taken their own lives since our previous inspection in 2015 and, staggeringly, 19 prisoners had taken their own lives at the establishment since 2011.
"Tragically, a few months after this inspection another prisoner was reported to have taken his own life."
At the time of the unannounced visit in February, there were 55 officer vacancies and 20% of officers had less than 12 months' experience.
Inspectors found a "decidedly mixed" picture - the assessment of respect for prisoners and rehabilitation and resettlement work were both "reasonably good."
Safety and purposeful activity were both assessed as poor, the lowest assessment, and had deteriorated since 2015, with nearly a third of prisoners saying they felt unsafe.
HM chief inspector of prisons Peter Clarke said incidents of self-harm "remained high" and although there had been improvements to the way prisoners at risk of self-harm were assessed and supported, "not all planned improvements had been sustained".
Mr Clarke said "chronic staff shortages and inexperience" underpinned nearly all of the concerns raised, adding that a "disappointingly small number of recommendations from our previous inspection had been achieved".
Chief executive of HM Prison and Probation Service, Michael Spurr, said the prison manages a complex and vulnerable population" and "remains focused on safety and supporting vulnerable men". He said new recruits would increase staff numbers in the coming months.