Rio de Janeiro — Brazil’s Rio de Janeiro state has unlawfully extended an unpaid labour scheme for prisoners denounced by human rights experts as a form of modern slavery, official documents obtained exclusively by the Thomson Reuters Foundation reveal.

Brazil’s penal system has long relied on cheap prison labour in its overcrowded, underfunded jails, with inmates cooking, cleaning and maintaining quarters in return for about 748 reals ($145) each month, and slightly reduced sentences...