Jails are "full to bursting" and the Government must "be brave" and cut the numbers behind bars, the president of the Prison Governors Association will warn on Tuesday.
Andrea Albutt will call on ministers to reduce the prison population in England and Wales, saying they should not "worry about votes".
In a highly critical assessment, she will describe violence, suicide and self-harm statistics as "the worst we have ever seen" and raise the alarm over staffing levels.
Addressing the association's annual conference, Ms Albutt will also say a safety and reform drive is "led and predominantly run by generalist civil servants with little or no understanding of the very complex nature of prisons and their inhabitants".
Ms Albutt's intervention will lead to fresh scrutiny of the politically-charged issue of the prison population.
Penal reform campaigners frequently warn that efforts to stabilise the estate will fail unless the number of inmates is reduced.
On Friday the prison population stood at 85,375, which is 1,124 below the "useable operational capacity". The level has almost doubled since the early 1990s and remained around the mid 80,000s mark in recent years.
Ministers have resisted calls for direct measures designed to bring about an immediate cut in the prison population, instead focusing on driving down re-offending rates and improving confidence in non-custodial punishments.
Ms Albutt will say: "Currently our prisons are full to bursting. The Government must be brave and reduce the prison population and don't worry about votes. Don't dabble, just do it - because morally it is the right thing to do."
Describing sentences of a year or less as "pointless", she will say: "This cohort must be dealt with in a different way in the community.
"Executive Release is possible. We have prisoners on IPP (Imprisonment for Public Protection) sentences years past their tariff but still in prison."
Jails are holding "old and infirm" inmates who are no longer a danger to society, as well as "far too many mentally ill people where prison is absolutely the worst place for them", she will tell conference.
Ms Albutt will express doubts over a Government commitment to provide 10,000 new prison places by 2020, describing the target as a "distant dream".
She is also expected to warn that staffing issues mean prisons are unable to deliver a rehabilitative regime.
The Government has launched a drive to add 2,500 new frontline officers and, although Ms Albutt will acknowledge that the picture is improving, she will also flag up the "attrition rate" in staffing levels.
A Prison Service spokesman said: "The Justice Secretary has been clear that our wide-ranging prison reforms will continue unabated and we will tackle the challenges facing the estate head on...
"By next year every governor will be able to tailor education and training to the needs of their prisoners, providing offenders the right support and challenge to help turn their backs on crime."