Andy Burnham 'should put his money where his mouth is and vote with the Conservatives', the Tory leader declared
Kemi Badenoch has called on Labour to ditch "abhorrent" plans to release sex offenders early.
Her party will force a Commons vote in an effort to block the release of rapists and sex offenders in a scheme by the Government to free up space in overcrowded jails.
The Tory leader has challenged Andy Burnham to vote with her party, days after it was revealed the Makerfield MP was alarmed by the law.
Mr Burnham is looking into how to change the law to ensure child groomers and rapists are unable to be released, according to The Times.
Ms Badenoch wrote in the Daily Mail: "Andy Burnham has said he doesn't think sexual abusers should be released early.
"He is about to become Prime Minister. If he's serious about standing up for victims then he should put his money where his mouth is and vote with the Conservatives, and tell the MPs courting his favour to do the same."
The Conservatives previously tabled an amendment to prevent rapists, paedophiles, and sex offenders from being awarded automatic early release.
"We warned the Government their Sentencing Bill would do this, but they didn't listen," Mrs Badenoch said.
Ahead of the debate, the Conservatives launched a petition at DontFreeRapists.com calling on the Government to re-think plans.
Shadow Justice Secretary Nick Timothy said Labour had promised early release would not apply to the "most serious, heinous crimes" but had now "broken their word".
Most criminals, including some sex offenders, will be eligible for release after serving just a third of their sentence and if they have demonstrated "good behaviour" while behind bars.
Some offender convicted of sex crimes will be made to serve half of their sentence - down from the current two-thirds.
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More than 7,300 victims have been warned by the Ministry of Justice their offenders could qualify for early release.
The first wave of criminals will be released beginning in early September.
Mrs Badenoch will met with six rape and sexual assault victims who were exploited by grooming gangs on Tuesday.
Among them are Fiona Goddard, a victim of a sex child grooming, abused when she was living in a children's home in Bradford when she was 14.
She said: "We spent years fighting for justice, reliving the worst things that happened to us, only to be told by email that violent offenders may be released early because prisons are overcrowded."
She added: "I always knew these men would be released one day, but I believed I had more time to rebuild my life and prepare myself for that reality.
"Finding out that they may be released early has completely set me back.
"I have barely left the house. I feel constantly anxious, frightened and unsafe. I can barely sleep."
A separate petition - launched on Change.org - also called for a reversal of the Sentencing Bill, has received over 60,000 signatures.
A Ministry of Justice spokesman said: "This Government is fixing the prison crisis it inherited – building 14,000 more prison places and reforming sentencing so we can always lock up dangerous criminals.
"Without this decisive action, prisons will run out of space entirely as early as November and we will be unable to lock up serious offenders at all."
