Prison numbers cannot be cut with "dangerous quick fix" solutions, Justice Secretary Liz Truss will say.
The Cabinet member will use a keynote address on Monday to attack a "reckless" opposition call to reduce the current number of 85,000 prisoners in England and Wales to the 1990 level of 45,000.
Ms Truss will say: “Reductions by cap or quota, or by sweeping sentencing cuts are not a magic bullet, they are a dangerous attempt at a quick fix."
The Justice Secretary says Britain needs to deal with the reality of the present, rather than hark back to prison numbers of the past.
"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."
Ms Truss says targeting sexual offenders more effectively has pushed the prison population up.
“The biggest driver for prison growth in the last 20 years has been the exposure, pursuit and punishment of sexual offences, domestic abuse and other crimes of violence," she will say.
Ms Truss says the Government is dealing with human "wickedness" as she attacks Labour for its stance.
“Shadow attorney general Shami Chakrabarti wants to halve our prison numbers because she does not believe 'the sum of human wickedness' could have doubled in her lifetime.
“She blames a political arms race for the numbers in our jails.
“What has actually happened in Baroness Chakrabarti’s lifetime is that the criminal justice system has got better at catching and convicting criminals who have perpetrated some of the most appalling crimes imaginable.
"And sentence lengths now better reflect the severity of crimes like domestic violence, rape and child abuse.
“It’s not that the sum of human wickedness has doubled – it’s that we have driven that wickedness out from the shadows and put it where it belongs, behind bars.”