The Chief Inspector of Prisons has demanded immediate action from the Government to improve conditions at a jail hit by soaring levels of violence and self-harm.
Peter Clarke put Justice Secretary David Gauke on notice that he must explain how conditions at HMP Exeter will be improved.
Mr Clarke's intervention is the second time he has triggered an alert using a scheme set up last year to allow the watchdog to raise the alarm quickly after an inspection.
An unannounced inspection of HMP Exeter took place between May 14 and 24.
Mr Clarke told Mr Gauke the principal reasons for invoking the urgent notification mechanism were that safety in the prison has "significantly worsened in many respects" since the previous inspection in August 2016.
Inspectors found disturbingly high levels of violence and self-harm and a serious failure to tackle safety issues.
Mr Clarke said: "There have been six self-inflicted deaths, five of which were in 2017.
"Despite some creditable efforts to implement recommendations from the Prisons and Probation Ombudsman following those deaths, the overall level of safety at HMP Exeter is unequivocally poor."
Assaults against both prisoners and staff were among the highest inspectors had seen, illicit drugs were rife and living conditions for too many inmates were unacceptably poor, the inspectorate found.
Prisoner-on-prisoner assaults had gone up by 107% since the last assessment, while attacks on staff rose by 60%.
Mr Clarke reported that there was a "strong smell" of drugs on some of the wings and he saw inmates who were "clearly under the influence" during the inspection visit.
Many cells were in a very poor state of repair, with broken windows, leaking lavatories and sinks and poorly screened toilets.
In one case a vulnerable prisoner assessed as being at a heightened risk of suicide was seen in a "squalid" cell without bedding or glass in his window.
Mr Clarke said: "The senior management team that is currently in place at HMP Exeter is largely the same as at the last inspection in 2016.
"The failure to address the actual and perceived lack of safety, and the issues that contribute to both, is so serious that is has led me to have significant concerns about the treatment and conditions of prisoners at HMP Exeter and to the inevitable conclusion to invoke the UN (urgent notification) protocol."
Under the urgent notification process, the chief inspector can inform the Government of any urgent and severe prison problems found during an inspection.
The Justice Secretary then has 28 days to publicly report on improvement measures adopted at the jail in question.
Mr Clarke's intervention on HMP Exeter marks his second use of the scheme since it was introduced in November.
The first urgent notification was issued in January after inspectors assessed HMP Nottingham as "fundamentally unsafe".
Prisons Minister Rory Stewart said: "I am grateful to the Chief Inspector for identifying the urgent attention required at HMP Exeter - and I am determined that we act immediately.
"Staffing has increased at Exeter and we expect to see improvements as a result.
"We will provide all the additional support needed to improve safety and reduce self-harm and we are already conducting a rapid review of conditions to improve the standard of cells."
Built in 1853, HMP Exeter is a category B facility for male inmates and has an operational capacity of 544.