The rising pressure on prisons from budget cuts and increasing numbers can not go on indefinitely, the Chief Inspector of Prisons has warned.
Nick Hardwick said there was a "critical issue" over how fast the population rose before two new prisons open in the next couple of months.
The prison population in England and Wales hit 87,583 on Friday, less than 600 off the all-time high of 88,179 set last December and 98% of the usable operational capacity. The rise has been fuelled largely by the tough sentences handed out in the wake of last summer's riots.
Mr Hardwick told the Financial Times: "We are asking prisons to do a very difficult thing. On the one hand numbers are going up and money is going down. And we're saying, on top of that, we want you to do more (to prevent reoffending). I think that's actually not an unreasonable ask for a period of time... But I don't think that can go on indefinitely."
He went on: "Can the rise be slow enough so that it doesn't reach 100% before the new (prisons) come on? What we don't know is whether the rise... is a hump that arises from the riots that will work its way through the system... or whether that's a long-term trend."
The latest figures released by the Ministry of Justice last week showed almost 1,000 people have been jailed for an average of more than a year following last summer's riots. Judges handed out sentences almost four times as long as for those convicted of similar offences in 2010.
Some 2,710 people had appeared before the courts by the start of last month following the looting and violence which spread across English cities last August, the figures showed.
A total of 945 of the 1,483 found guilty and sentenced for their role in the riots were jailed immediately, with an average sentence of 14.2 months.
This was much higher than the average 3.7-month sentence handed down to those convicted by magistrates but sentenced at any court for similar offences in England and Wales in 2010, the figures showed.
Read More
Overall, more than half (53%) of those before the courts over the riots were aged under 20, half (49%) were facing burglary charges, and one in five (21%) were accused of violent disorder.
