United States presidential candidate Michael Bloomberg said that his campaign "unknowingly" used prison workers to call voters on his behalf.
The billionaire Democrat said he had cut ties with a vendor which operates two call centres in state jails.
Bloomberg said he learned about the company's use of prison labour only after receiving a call from a reporter.
He entered the election race in November with one of the largest-ever political ad buys.
His campaign's use of prison labour was first reported by news site The Intercept, and confirmed by Mr Bloomberg in a statement on Tuesday.
"We do not support this practice and we are making sure our vendors more properly vet their subcontractors moving forward," said the former New York mayor.