US presidential candidate Michael Bloomberg has admitted his campaign used prison labour to make canvassing phone calls.
One of the frontrunners in the race to take on Donald Trump in the election next year, he said he only found out when a journalist uncovered it.
The Intercept had reported Democrat Mr Bloomberg's campaign was contracting campaign calls to voters in California to ProCom – a third-party company with a call centre based in Oklahoma, Texas.
It revealed ProCom was using prisoners in the Dr Eddie Warrior Correctional Center – a minimum-security women's jail with a capacity of more than 900.
The website said that while ProCom paid the minimum wage of $7.25 per hour to the state's department of corrections, the department caps prisoners' pay at $20 a month.
Callers did say at the end of their conversations they were calling on behalf of the Bloomberg campaign but did not disclose they were speaking from behind bars, it added.
Mr Bloomberg said the story was "fundamentally accurate" but "we only learned about this when the reporter called us".
He confirmed that as soon as he found out which company was responsible, the campaign "immediately ended our relationship" with them.
"We do not support this practice and we are making sure our vendors more properly vet their subcontractors moving forward," the former New York City mayor added in a statement on Twitter.
The use of prison labour was condemned by Alex Friedmann, managing editor of Prison Legal News.
He said: "The use of prison labour is the continued exploitation of people who are locked up, who really have virtually no other opportunities to have employment or make money other than the opportunities given to them by prison officials."
John Scallan, a ProCom co-founder, was quoted by The Intercept as saying of the $20 prisoners' pay cap: "I can tell you unequivocally that is not us. Some of them are making that much every day."
Mr Bloomberg is fighting other Democrat politicians to be the party's official nominee in the 2020 presidential election, taking place on 3 November.
It comes after a six-year-old girl in the UK found a message from a prisoner in China inside a Tesco Christmas card.
The note read: "We are foreign prisoners in Shanghai Qingpu prison China. Forced to work against our will. Please help us and notify human rights organisation."
Tesco said it was shocked by the find and had started an investigation, and has also stopped working with the Chinese factory where the card – decorated with a kitten wearing a Santa hat – was made.