The number of suicides at prisons in England in Wales in 2013 was the highest for six years, the Howard League for Penal Reform has said.
In total, 199 deaths were reported, 70 of which were self-inflicted.
Howard League chief executive Frances Crook said almost all deaths in custody were "preventable".
The Prison Service said it was committed to safety and each death was investigated by police and the Prisons and Probation Ombudsman.
The Howard League statistics are based on notifications from the Ministry of Justice.
Ms Crook said: "Each death in custody is a tragedy and almost all of them are preventable.
"The responsibility for an increase in the number of people who take their own lives in prison lies squarely with those who advocate putting behind bars more and more people who do not need to be there."
The Howard League said there were four murders or suspected murders in prisons in 2012, the highest for 15 years.
Last year inmates at HMP Hewell and HMP Long Lartin, both in Worcestershire, were murdered.
More than 100 prisoners died of natural causes, and a further 22 deaths are yet to be classified by prison authorities, the charity said.
Ms Crook added: "This is the consequence of a policy that squanders a scarce resource, meaning that these institutions cannot keep people safe."
The Prison Service said it was "applying strenuous efforts to learn from each [death]".
Five suicides were recorded at Wormwood Scrubs in London, while there were four at Woodhill, near Milton Keynes.
A further six prisons each recorded three self-inflicted deaths during the year.
They were Hewell, Birmingham, Dovegate, Norwich and Chelmsford.
There were 50 prisons where no death was recorded and 19 where all the deaths were by natural causes.