Appalling levels of suicide and self-harm behind bars are a "damning indictment" of the state of prisons, a Commons committee has warned.
In a highly critical assessment, MPs argued there are "deep-rooted failures" in the management of prisoners' mental well-being.
The deteriorating prison estate and long-standing under-staffing have created an environment which exacerbates the mental health issues faced by prisoners, according to the Public Accounts Committee.
It also flagged up the increased availability of drugs in jails.
The committee's report claimed the Government has no reliable or up-to-date measure of the number of inmates who have mental health problems.
Existing screening procedures were said to be insufficient to adequately identify those in need of support and treatment.
Committee chairwoman Meg Hillier said: "There are deep-rooted failures in the management of prisoners' mental health, reflected in what is an appalling toll of self-inflicted deaths and self-harm.
"Failing to attend to the mental health needs of inmates can also have devastating effects beyond the prison gates.
"The evidence is stark but there is no realistic prospect of these serious issues being properly addressed unless Government rethinks its approach.
"This must start with a meaningful assessment of the scale of the problem.
"Without adequate data it is simply not possible to determine whether Government action is making a difference - yet, incredibly, the most commonly used estimate of prisoners' mental health problems is 20 years old.
"This is clearly not good enough and implementing more robust health-screening processes must be a priority."
The latest official figures show there were a record 41,103 reported incidents of self-harm in prisons in England and Wales in the 12 months to June 2017, up 12% from the previous year.
There were 77 self-inflicted deaths in jails in the year to September, which was down 33 on the previous year.
Professor Pamela Taylor, chairwoman of the forensic faculty at the Royal College of Psychiatrists, said the Government "must look harder at how to prevent so many people with mental health problems ending up in prison".
She added: "Many prisoners have severe mental disorders but are treated in an entirely inappropriate environment."
A Ministry of Justice spokeswoman said: "Every death in custody is a tragedy and we are redoubling our efforts to support vulnerable offenders, especially during their first 24 hours in custody.
"All prisoners are subject to health screening when entering prison and their mental health is monitored closely while they serve their sentence.
"In April this year we introduced new suicide and self-harm reduction training - over 11,000 staff have embarked on the new training.
"We continue to support the prisoner listener scheme, as well as providing extra funding for the Samaritans.
"We will continue to work closely with NHS England to improve services in a number of areas, including the process for prisoners who require transfer to secure hospitals.
"We have been clear that improving safety in our prisons is our priority - that is why we are investing £100 million to increase staffing by 2,500 officers and we are taking unprecedented action to tackle drug use which undermines safety and stability."
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