Campaigners hit out after the blistering verdict from the National Audit Office.
Ministers do not know how many prisoners are mentally ill, fuelling a record high in jail suicides, a Whitehall watchdog reveals today.
Self-harm behind bars has soared in the last five years, suggesting mental health and wellbeing among inmates has plunged, the National Audit Office warns.
Self-harm rose by 73% between 2012 and 2016 with 40,161 incidents of self-harm in prisons recorded last year.
There were 120 prison suicides last year – nearly double the number in and the highest death toll on record.
And while 31,328 inmates report having mental health problems at any one time, the true figure is unknown.
The report blasts: “Government does not collect enough, or good enough, data about mental health in prisons, which makes it hard to plan services and monitor outcomes.
“There are no reliable data on the prevalence of mental illness in prisons.
“The most commonly used estimate is that 90% of the prison population are mentally unwell, but this figure dates from 1998 and uses a broader definition of mental illness than many clinicians would recognise.