US presidential candidate Michael Bloomberg has 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. Mr 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...