Until 1970 it was technically possible to be imprisoned for being in debt and unable to pay. During the 18th and early 19th centuries more than half of all prisoners were debtors. In some places, e.g. the Fleet in London, there were separate prisons for debtors, but in most local prisons the debtors were simply kept apart from other prisoners in their own wings. They were allowed more visitors than convicts, and were released if they could find someone to pay their debts, or if they could earn enough in prison to settle them.
Until the law changed in 1815, debtors could be worse off after a few years in prison than when they entered. Having no money had brought them to prison in the first place; having to pay for their keep put them farther into debt. The small amounts that they could earn in prison were usually not enough to cover their keep.
Debtors did not have to do hard labour. Sometimes the rules were applied loosely: In 1814 the gaoler in Bedford admitted that he had used a debtor as "turnkey" (prison guard), for which, along with other offences, he was sacked. The 1823 Prisons Act stated that debtors should not be made to work without their consent, and should never be made to work on the tread wheel.
Some debtors were imprisoned through no fault of their own: the Bedford gaoler in 1858 complained that 73 out of the 122 debtors sent to the gaol owed money to hawkers (door-to-door salesmen) who had left goods with them without being asked to do so, then took them to court when they would not or could not pay.