Over half the prisons in England and Wales are officially overcrowded, a penal reform group claimed as prison numbers reached an all-time high.
The Home Office predicts prison numbers will rise to 90,000 by 2010
With a record prison population of 76,897, 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, it added.
The Home Office said no prison operated at above safe population limits and that the figures were misleading.
According to Home Office projections, the prison population could top 90,000 by the end of the decade.
And the trust claims that prisons are currently coping with 10,000 more prisoners than they were designed to hold.
Trust director Juliet Lyon said the government had become complacent about the problem of prison overcrowding and was now "breaching its own final buffer".
"This level of overcrowding poses a real and serious danger to prison and public safety.
"The summer holiday season usually gives prisons a respite while the courts take their break, instead the population is growing month on month.
"Even in the quietest months of the year, pressure is still building up within prisons."
A Home Office spokeswoman said some prisons may exceed what is termed their "certified normal accommodation" - the figure at which the prison operates comfortably.
But she said: "We do not operate above the operational capacity of any establishment.
"The operational capacity is the total number of prisoners which an establishment can hold without serious risk to good order, security and the proper running of the planned regime."
On some occasions, however, prisons were wrongly listed as having populations higher than their operational capacity, she said.
The reason for this was because some prisoners were actually absent on authorised absences, such as being out on temporary licence, but were still recorded as part of the prison population, she added.