The prison population of England and Wales should be cut to 45,000, a former deputy prime minister and two former home secretaries have urged.
In a letter to the Times, Nick Clegg, Ken Clarke and Jacqui Smith said jails had become unacceptably dangerous.
They said inmate numbers had risen to more than 85,000 since Michael Howard declared in 1993 that "prison works".
The cross-party trio pointed out that almost half of inmates are re-convicted within a year of being released.
Former Lib Dem leader and deputy PM Mr Clegg, ex-Conservative Home Secretary Mr Clarke and former Labour Home Secretary Mrs Smith said the recent violent unrest at Birmingham Prison was a "wake-up call for this country".
Riot teams restored order to all four wings of HMP Birmingham last week after more than 12 hours of rioting described as the worst since events at Strangeways 26 years ago.
'Order, security and purpose'
The trio's letter said there had been a 31% increase in prison assaults in the past year, and one prisoner killed themselves every three days.
"We believe that an escalating prison population has gone well beyond what is safe or sustainable," they wrote.
"To restore order, security and purpose to our jails, ministers should now make it their policy to reduce prison numbers.
"If the tide is not turned soon, the prisons crisis will do untold damage to wider society."
Mr Clegg told BBC Radio 4's Today programme that it was "with the benefit of hindsight and in a non-partisan way" that he, Mr Clarke and Mrs Smith were able to make the suggestions.
"All Ken and Jackie and I are saying is yes, if we chose to as a country, we could move to American style levels of mass incarceration - it's very expensive, it's very ineffective - or, we can try and think again and make our prison estate smaller, safer and more effective in dealing with crime," he said.
Cutting the number of inmates to 45,000 would bring the prison population close to what it was under Margaret Thatcher's Conservative government, the letter said.
"The system is not serving victims of crime or properly protecting our communities either," it said, but did not explain how the three thought the reduction could be achieved.
Justice Secretary Liz Truss has said violence in prisons "will not be tolerated".
'Fearful nearly every day'
Amid claims that inmates in Birmingham were pushed to riot by poor conditions and a lack of staff, a former officer at the jail has spoken anonymously about conditions there.
"I started work at the prison more than 10 years ago. It was my first prison job and I was a fresh recruit.
I felt safe at first. During the first few years I could count the amount of times I felt fearful for my own safety on one hand. Eventually it got to the point where I was fearful nearly every day.
At first, if you were dealing with an incident you would have colleagues around you.
But gradually I realised they were taking more time to turn up, mainly because they may be coming in from other areas.
It's a psychological process - you become more wary of challenging prisoners so prisoners become more confident and less respectful."