Labour will "in all likelihood" need to continue the release of early prisoners as a result of overcrowding in jails.
The prison population has inflated in recent years due to tougher sentences and court backlogs, though Sir Keir Starmer blamed the Tories for not building enough new ones.
The policy that was introduced in October last year allowed some less serious offenders to be released up to 18 days early. This was extended to 70 days in May.
"In all likelihood, we will have to continue with that," Sir Keir told the BBC.
He added: "I have to say it is shocking to have to inherit a problem like that.
"That our criminal justice system has gotten to a point where we're releasing prisoners who should be in prison early and giving instructions to the police not to arrest in certain cases.
"That is how broken the system is. We have to pick that up and start the fix - not just a fix, but to renew and take forward."
Meanwhile, the Conservatives have defended their record, saying they have delivered 6,000 new prison places as part of "the largest expansion of the prison estate since the Victorian era".
Their 2030 manifesto promises to build four new prisons, in turn creating 20,000 new places, by 2030.