With the prison population standing at 76,897, figures from the Prison Reform Trust show that 74 of 142 jails are operating at over the government's certified occupancy level.
And the trust claims that prisons are currently coping with 10,000 more prisoners than they were designed to hold and that in 15 cases, jails have too many inmates.
The Home Office predicts that prison numbers will rise to 90,000 by 2010.
The 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," she said.
"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 that no prison operated at above safe population limits and that the figures were misleading. On some occasions prisons were wrongly listed as having populations higher than their operational capacity, she said.
The reason for this was because some prisoners were actually out on authorised absences - such as being out on temporary licence - but were still recorded as part of the prison population, she added.