The prison population in England and Wales reached a record high today for the second consecutive week.
The total reached 85,495 - 127 more than the previous record of 85,368 set last week and just more than 2,000 short of the usable operational capacity of 87,638, figures released by the Ministry of Justice showed.
Criminal justice campaigners will say the figures are a warning for the Government that it must act urgently to reduce the number of people in prison.
Justice Secretary Kenneth Clarke wants a greater emphasis on rehabilitation and community sentences as he reviews sentencing and penalties, saying the prison population is too high.
Geoff Dobson, deputy director of the Prison Reform Trust, said: "Ken Clarke has the opportunity at Conservative Party conference to set out how he intends to reverse the unsustainable trend in ever-expanding prison numbers.
"He could call a halt to this upward drift by ensuring that petty offenders do enforced community work, people who are mentally ill are diverted into healthcare, and addicts into treatment."
Roma Hooper, director of Make Justice Work, said: "Locking up thousands of low-level offenders for short periods of time has certainly contributed to the flood of new prisoners.
"Ken Clarke has broken the dam by admitting short-term prison sentences don't work. Now Ed Miliband has said he agrees.
"There is no excuse and today's figures are a final warning: we must act now or pay the tax and social costs of a penal system stretched beyond its limits."
The Ministry of Justice said long-term decisions on the number of prison places available will be made following the Government's review of sentencing and penalties for offenders.
"We will look in detail over the coming months at the sentencing frameworks for adult and young offenders, as well as the full range of penalties available in the criminal justice system," a spokesman said.
"This means introducing more effective policies, as well as overhauling the system of rehabilitation to reduce reoffending.
"We will take time to get it right and will consult widely before bringing forward coherent plans for reform.
"Long-term decisions on prison capacity programmes will be taken in the light of these policy developments.
"We will ensure that we meet prison capacity requirements more efficiently to improve value for money for the taxpayer and contribute savings to help reduce the budget deficit."