Thousands of prisoners are to be released early from September in order to prevent the "collapse" of the prisons system, the UK's new justice minister announced on Friday.
Shabana Mahmood said failure to do so risked the "total breakdown of law and order", with just 700 places left for men, and jails operating at 99 percent capacity since 2023.
England and Wales have the highest per capita prison population in Western Europe.
The chief inspector of prisons, Charlie Taylor, said this week that urgent action was required, as prisons were at "breaking point".
The release initiative would not apply to violent offenders serving more than four years, sex offenders and those in prison for domestic abuse offences as well as those on life sentences.
Mahmood, appointed last week after Labour's general election win, warned in dramatic language that if prisons ran out of cell space, there could be "van-loads of dangerous people circling the country with nowhere to go".
"With officers unable to act, criminals could do whatever they want, without consequence. We could see looters running amok, smashing in windows, robbing shops and setting neighbourhoods alight," she said in a speech at a prison.
"In short, if we fail to act now, we face the collapse of the criminal justice system."
Mahmood accused the previous Conservative government of a "dereliction of duty" regarding the system.
Her comments echoed Prime Minister Keir Starmer, who earlier told reporters while attending a NATO summit in the United States that the ousted Tories were guilty of "gross irresponsibility".
"The scale of the problem was worse than we thought," he said.
Mahmood's Conservative predecessor in the post, Alex Chalk, had reportedly pressed prime minister Rishi Sunak to impose similar measures before the election, but was overruled.
Under Labour's plans, prisoners who are eligible for automatic release after serving half their sentence will be freed earlier than the normal.
The plans involve a temporary reduction of how long prisoners must serve behind bars, from 50 percent to 40 percent of their sentence.
The releases will start in September to give the Prison and Probation Service time to plan.
Mahmood also announced the recruitment of 1,000 additional trainee probation officers by next March.
The total number of prisoners hit over 87,505 on Friday -- of whom 83,755 were men -- leaving just 1,451 spaces available, official figures showed.
Since the start of 2023, the average occupancy rate in men's prisons had "routinely been higher than 99 percent", according to the Ministry of Justice.
Officials say the prison system needs a buffer of around 1,425 cell spaces in men's prisons at all time to deal with sudden influxes of inmates.
Six new prisons to create an extra 20,000 places are being built -- of which about 10,000 will be ready by the end of next year.
The Prison Governors' Association (PGA) said it welcomed the speed of the new measures while demanding a "full review" and that "the public must never be placed in this position again".
PGA chairman Mark Fairhurst said the measures would free up around 4,500-5,000 more spaces and give prisons around 12 to 18 months before further steps would be needed.
Meanwhile Prison Reform Trust chief executive Pia Sinha said ministers "had the good sense to listen to the advice of its officials and introduce further emergency measures".
