More than half of prison suicides occur in the most overcrowded jails, according to new research.
The Prison Service says suicide rates are falling this year
Since the start of 2004, 56% of prison suicides happened in England and Wales' 35 most overcrowded jails, according to the Howard League for Penal Reform.
The charity's director, Frances Crook, described overcrowding as "the canker at the heart of the penal system".
The Prison Service said the overall number of inmates committing suicide "has been falling over the last year".
The Howard League's findings were outlined ahead of a House of Lords debate on the impact of overcrowding on inmate suicides.
Opening the debate, Liberal Democrat Lord Dholakia - formerly a member of the Howard League's council - called for fewer custodial sentences.
"We should aim to send fewer people to prison and we need a determined effort by politicians to secure a shift in the public perception of crime and punishment," Lord Dholakia said.
"We should only send to prison people whose offence makes any alternative unacceptable.
"Those who are sent there should not stay there longer than necessary."
Last week the Prison Reform Trust said 74 of 142 jails were operating at over the government's certified occupancy level and 15 had too many inmates to meet safe overcrowding limits.
The prisons with the highest number of suicides - Gloucester, Norwich and Manchester, with seven deaths each - all had about a third more inmates than their normal maximum capacity.
About 159 suicides were recorded since the start of last year in the 142 prisons of England and Wales.
Preston and Shrewsbury, the most overcrowded jails, each held 180% of their certified capacity.
Both had experienced three and five suicides respectively since January 2004.
The research also revealed that the majority of suicides occurred in local prisons, which house suspects remanded in custody as well as convicted offenders.
And more than half of all prison suicides are by remandees, despite them making up less than 20% of the total population.
Ms Crook said some 17,200 prisoners were forced to "double up" in cells designed for one, as the prison population rose, reaching a record 77,702 last week.
She said: "The Howard League for Penal Reform believes that instead of locking up ever-increasing numbers of men, women and children in jails that have no room for them and can do nothing with them, the courts should take advantage of effective community sentences.
"If we do not halt the catastrophic increase in imprisonment, then we are merely creating further victims both inside and outside prison."
A Prison Service spokeswoman said: "So far this year there have been 64 self-inflicted deaths in prisons, three of which were women, whilst at this time last year there had been 85 such deaths, including 13 women.
"This 25% reduction has taken place against a background of a record prison population."
She said "annual numbers can be volatile", adding that the three-year rate was "a much more reliable indicator of underlying trend" and suggested the number of suicides "has been falling over the last year".