The Prison Reform Trust (PRT) said measures allowing thousands of criminals to get out of jail 18 days early had only bought ministers a "brief respite" from the overcrowding crisis.
There were 81,040 inmates in England and Wales when the emergency measures were introduced on June 29, but the total has already crept back up to 80,708. Last Friday, a week-on-week rise in the prison population of 389 was recorded. If that is repeated this week, the record will be broken.
Juliet Lyon, the PRT director, said: "The government is as little as a week away from losing entirely the brief respite in prison numbers gained by early release.
"Even over the summer, when many courts are away, the prison population has been growing by almost 400 a week - the size of a small prison.
"With the government's planned prison building programme years away, there is no time for it to build its way out of a renewed crisis before the autumn. The only way out is to take a more sensible, effective approach to crime and punishment.
"This means reserving prison for serious and violent offenders, adjusting the sentencing framework to allow for proper judicial discretion, cutting any unnecessary recalls and remands, making sure that petty offenders do enforced community work and diverting the mentally ill and addicts into treatment."
A Ministry of Justice spokeswoman said: "The National Offender Management Service is dealing with pressures on the prison estate by building more capacity.
"In the last two years, there has been an increase of around 3,200 places which includes building additional places at existing prisons and the opening of a new prison.
She said a new capacity-building programme would deliver 8,000 new prison places by 2012, on top of already planned expansions at existing prisons, which would add about 700 places during 2007.