Michael Bloomberg's presidential campaign used prison labor to make calls for his candidacy, it was revealed on Tuesday.

The billionaire's campaign used a third party vendor to contract with New Jersey-based call center company ProCom, which uses incarcerated people in state prisons to make phone calls, The Intercept reported.

In at least one prison in Oklahoma - the Dr. Eddie Warrior Correctional Center, a minimum-security women's prison with a capacity of more than 900 - prisoners were contracted to make calls on behalf of the Bloomberg campaign.

Michael Bloomberg's campaign used prison labor to make calls for his candidacy

Bloomberg's campaign used incarcerated people - like in this photo of California prisoners in 2010 - to make calls for his campaign

The callers were legally required to say the calls were paid for by Bloomberg's campaign but did not have to reveal they were calling from prison.

Bloomberg's campaign confirmed the arrangement to the news outlet.

'We didn't know about this and we never would have allowed it if we had. We don't believe in this practice and we've now ended our relationship with the subcontractor in question,' Bloomberg spokesperson Julie Wood said.

Bloomberg, who is worth more than $50 billion, entered the presidential contest in November.

He's not accepting campaign donations but funding his own presidential bid.

Some of his Democratic rivals have accused him of using his fortune to buy the presidency.

His campaign ended its contract with ProCom on Monday, when The Intercept inquired about the relationship.

It's unclear how much the prisoners were paid for their work or what type of calls they made.

John Scallan, a ProCom co-founder, told the news outlet his company paid the Oklahoma minimum wage of $7.25 an hour to the Oklahoma Department of Corrections, which then pays the prisoners working in the call centers.

The Dr. Eddie Warrior Correctional Center in Ohio is one of the prisons used to do the work

The Oklahoma Department of Corrections website lists the maximum monthly wage at $20 dollars a month, but another state document says it's $27.09 per month.

'I can tell you unequivocally that is not us,' Scallan said. 'Some of them are making that much every day.'

The Oklahoma Department of Corrections did not answer Intercept's request for comment.

Bloomberg has focused his campaign strategy of skipping the early voting states of Iowa and New Hampshire for the later Super Tuesday contest - where big states like California will have their large delegate populations up for grabs.