Suicides in English and Welsh prisons in 2004 have matched the highest level recorded two years ago, Home Office figures show.
The number of prison suicides is back to record levels
Ninety-five inmates killed themselves, including 13 women - a higher proportion than among male prisoners.
This month, MPs warned jails were too often used to hold those with a history of mental illness and addiction.
Prison Reform Trust Director Juliet Lyon warned against "turning prisons into low-level psychiatric hospitals".
She added: "These tragic deaths should shock but not surprise.
"It is unreasonable to expect a few hard-pressed staff doing their best in bleak prisons to cope with vulnerable people in urgent need of mental health treatment and care.
"We should be looking at investing in court diversion and liaison schemes, improved psychiatric services and more medium secure hospital beds."
Overcrowding has also been blamed in the past for prison suicides, but the rising prison population stabilised this year, says a BBC correspondent.
The figure of 95 includes two inmates who were found dead in their cells this week.
Remand prisoner Carl Dunn was found by prison staff hanging from bedsheets at HMP Bullingdon near Bicester, Oxfordshire, on Wednesday.
On Monday, Dennis Williams, 23, was found hanging in his cell in Bedford Prison.
Earlier this month, the Joint Committee on Human Rights, made up of MPs and peers, said deaths in custody had risen to "shocking levels".
The report added: "The number attempting suicide is on average twice that of those dying in custody, and the number carrying out incidents of self-harm should be a cause of huge concern."
It urged the government to set up a cross-departmental group to stop deaths in prisons, police cells, special hospitals, immigration centres and other detention areas.
The Home Office said on Friday it was working closely with all the relevant government agencies, including the Department of Health and the Youth Justice Board, on tackling prison suicides.
"For example, the Prison Service is in partnership with the NHS and is working closely with NIHME [National Institute for Mental Health in England] in increasing the number of specialist mental health employees in prison," it said.
It was also working to hire more specialist mental health staff to deal with those deemed to be suicide risks, a spokesman said.
"Reducing suicides and self-harm in prison is a key objective and never before has more been done to reduce the number of self-inflicted deaths in custody."