Hundreds of prisoners must be released early to avoid “unchecked criminality” where the police and courts are unable to lock anyone up, Downing Street said.
Around 1,700 inmates are expected to be let out on Tuesday in an attempt to ease overcrowding in prisons.
Downing Street acknowledged it was “incredibly difficult” for victims to know that the offenders were being released before completing their sentence.
On Saturday, Sir Keir Starmer said he had been forced into the position because the Tory government had not built enough prison capacity.
A No 10 spokeswoman said: “As the Prime Minister said this weekend, no prime minister should be in a position where there are not enough prison places.
“That is why we took the immediate decision that we did. And without doing so, prisons would have completely run out of places, and we faced unchecked criminality on our streets with police unable to make arrests.”
She added: “This is another difficult, tough, but necessary action that we’ve taken.
“The approach that the Government’s taken is to ensure that we have safeguards in place so that we can protect the public, and we also recognise how incredibly difficult this will be for victims, and most importantly, under the release plans – which must be in place for every prisoner leaving – under those plans, victims will be told ahead of time. That is important.
“There are also exemptions to ensure that offenders for violent and serious offences are not eligible for release under the scheme.
“Similarly, everyone must be released with a release plan that includes having somewhere to live once they leave prison and support to find work.”
Under the scheme, some offenders will be released after serving just 40% of their custodial term.
Policing minister Dame Diana Johnson told the BBC: “For those who will be released tomorrow, probation have been working very hard to make sure that there are plans in place.
“People are released on licence and can be recalled if they commit further offences.
“So there’s a whole range of things that are already in place to make sure tomorrow goes as well as we can expect, but clearly nobody wants to be doing this.”
Asked how many prisoners were due to be released on Tuesday, she said: “I understand it’s in the region of about 1,700.”
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