Liz Truss has been accused of "mud-slinging" ahead of a major speech on prison reform at a London think tank on Monday (13 February).
The justice secretary is expected to claim Labour wants to "turn back the clock" and half the prison population in England and Wales from its current record level of 85,000 to 45,000.
"Reductions by cap or quota, or by sweeping sentencing cuts are not a magic bullet, they are a dangerous attempt at a quick fix," Truss will tell the Centre for Social Justice.
"There are those in Labour who want to turn back the clock and cut the prison population to the size it was in 1990, at around 45,000.
"This would be reckless and endanger the public. And it would restrict the freedom of an independent judiciary to choose the most appropriate sentence for each offender."
But Richard Burgon, Labour's shadow justice secretary, urged the senior Conservative to "take responsibility" and denied that a reduction to 45,000 was ever Labour's policy.
"Labour's policy is to properly fund and run our prison and probation service, make the safety of prison staff a top priority and make society safer by jailing those who are a threat to public safety. Labour will make rehabilitation work," he said.
"Conservatives in power have made a mess of our prison and probation service and have created the current prisons crisis.
"This is dangerous for staff, dangerous for inmates and dangerous for the public. We await with interest the Prisons and Courts Bill which will hopefully have some of the detail Truss' White Paper failed to provide."
The speech coincides with a new joint report between the Centre for Mental Health and the Howard League for Penal Reform. The charities urged the Government, among other things, to give prison staff more training so they can monitor inmates' well-being better.
The Ministry of Justice's own figures showed that suicides in English and Welsh prisons hit a record high of 119 in 2016, up 29 from the year before.
A number of prisons have also recently witnessed unrest, with a riot at HMP Birmingham in December. The Prison Officers Association has called on the government to increase staffing levels.