The NAO says that Clarke's sentencing reform programme, which included sentence discounts of up to 50% for early guilty pleas, was designed to reduce the size of the prison population by 6,000 by 2015.
Clarke had hoped to deliver a deeper cut in prison numbers of up to 6,000 but a crucial part of his penal reform package – his proposal to increase maximum sentence discounts from 33%to50%0% for early guilty pleas – was shelved in the aftermath of the row over rape sentencing.
Kenneth Clarke, the justice secretary, is expected to announce a climbdown over his plan for sentence discounts of up to 50% for early guilty pleas after he provoked a row by appearing to suggest that some rapes are more serious than others.
Cameron abandoned plans to offer a 50% sentence discount to offenders who submitted early guilty pleas after tabloid criticisms of "soft justice".
The original said that the Victims' Commissioner, Louise Casey, had described as "bonkers" the Kenneth Clarke proposal to increase the sentence discount for early guilty pleas from a third to half the sentence.
Clarke had told MPs on Tuesday it was "likely" that his proposal to increase the sentence discount for early guilty pleas from a third to half the sentence, including in rape cases, would be included in the final government plans.
Ministry estimates show 3,400 of the 6,000 fewer prison places that will be needed as a result of his sentencing package will come from the plan to increase the maximum available sentence discount from 33%to50%0%.
In the short term, proposals such as increasing the average sentence discount for a guilty plea from 25% to 34% would have the biggest impact, saving 3,400 prison places a year by 2014/15 and £130m.
An official impact assessment suggest it could save as many as 3,400 prison places a year by 2014 but reveals that officials expect the plan will only increase the average sentence discount of 25%to34%4%.
