Too many people are being sent to prison and the Government should be working "overtime" to reduce the size of the bulging jail population, former justice secretary Michael Gove has warned.
Delivering the annual Longford Lecture in London, he said effective rehabilitation was impossible as long as prisons remained overcrowded and underfunded.
Mr Gove - one of the leaders of the Brexit campaign who was sacked by Theresa May when she became Prime Minister - acknowledged he had "swerved" the issue while he was in office.
He also called for "executive clemency" for the estimated 500 prisoners still held under the controversial indeterminate sentences - which were scrapped four years ago - even though they had served the maximum determinate sentence for their offence.
"The problem - in a nutshell - is that we have a system operating at practically full capacity with nowhere near enough flexibility to devote the time, care and attention needed to secure successful rehabilitation," he said.
"It is an inconvenient truth - which I swerved to an extent while in office - that we send too many people to prison.
"And of those who deserve to be in custody, many, but certainly not all, are sent there for too long."
With half of ex-prisoners re-offending with a year of their release, Mr Gove said the system would not be able to deliver rehabilitation on the scale that was needed if far more was spent on prisons or inmate numbers were significantly reduced.
"I think we need to work, overtime and pragmatically, to reduce our prison population.
"We imprison a far higher percentage of our population than similar developed nations.
"And we have been sentencing individuals to significantly longer sentences over time in the last few years.
"In pragmatic terms, it seems to me obvious that we can provide a more effective rehabilitation regime with fewer offenders in jail and more professional attention given to those who remain.
"Because overcrowded prisons are more likely to be academies of crime, brutalisers of the innocent and incubators of addiction rather than engines of self-improvement."
Mr Gove also called for more prisons to become independent legal entities - along the lines of academy schools - with the potential for the best to take over under-performing institutions.