A spokesman for the Prison Officers' Association has said he was "concerned" that companies working with offenders were "paying pennies", and not the minimum wage.
Glyn Travis, assistant general secretary at the trade union, appeared on the breakfast show to talk about the news that more than 120 businesses had registered to work with prisons to help offenders improve their job prospects.
"We welcome the businesses who have said they're going to engage and work with the judiciary to try and get offenders back into work," Mr Travis told Julia Hartley-Brewer.
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"Our concern is that a lot of these companies are now starting to work with offenders whilst they're in custody and therefore they're not paying the minimum wage. They're paying pennies compared to what should be happening."
Mr Travis added that although employing offenders was "really key" to their rehabilitation, other groups who struggled to find employment should also be considered.
"You cannot get away from the fact that we've got a million people who are unemployed. Many of them have got disabilities, learning difficulties, who cannot get employment, and we believe that these companies should also be focussing on those individuals rather than the offender population because it's payment by results," he said.
"Offenders who come into prison have lost the trust of the public and the judiciary, and that's why they go to prison, but what we have got to do as part of the prison system and part of the judiciary is prepare them to go back into society."