Billionaire US presidential hopeful Michael Bloomberg used prison labor to make calls for his campaign, investigative website The Intercept reported on Tuesday.
Bloomberg called the report "fundamentally accurate" and said his campaign had already ended its relationship with the company involved.
The Intercept reported that the former New York mayor's campaign contracted, through a third-party vendor, the ProCom call center company based in New Jersey.
ProCom runs call centers in New Jersey as well as Oklahoma, where two of its call centers operate from state prisons, The Intercept said.
In at least one of those Oklahoma prisons, a minimum-security women's facility, inmates were contracted to make calls on behalf of the Bloomberg campaign, according to the report.
Bloomberg said his campaign only learned about this situation when the reporter on the story called them.
"But as soon as we discovered which vendor's subcontractor had done this, we immediately ended our relationship with the company and the people who hired them," he said in a statement.
"We do not support this practice and we are making sure our vendors more properly vet their subcontractors moving forward."
Months after frontrunners Joe Biden and Bernie Sanders, Bloomberg announced in late November his candidacy for the Democratic Party nomination to challenge Donald Trump in next year's presidential election.
Bloomberg has a net worth of more than $54 billion, according to Forbes.
He is using that personal fortune to spend heavily on online and TV ad campaigns, and his campaign is skipping crucial states long toiled in by less well-off rivals.
The Intercept said it asked Stephen McQuaid, a director of business development at ProCom, whether the company had done work for Bloomberg.
"To my knowledge we are not, and nor have I ever heard that we were making any dials for the Bloomberg campaign or on behalf of them through someone else, nor have I heard that we (are) currently making them, but I am not in a position to know every campaign that is going," he was quoted as saying.