A 37-year-old single mother of two children who is unemployed and being treated for depression and alcoholism is to fight against a one-month prison sentence for failing to pay a debt of €5,865.35 owed to Monaghan credit union.

AIB chairman Dermot Gleeson, also a senior counsel, will argue on the woman's behalf at the High Court later this month in what is regarded as a test case against the 1926 legislation which allows an individual be sent to prison for failing to pay a debt.

The critical High Court test comes as the latest figures show the number of people sent to prison for debt jumped by almost 40% last year, from 201 in 2007 to 276 last year.

The average sentence handed down was 27 days but in one case a sentence of 90 days was given to a person who failed to repay a debt.

In the test case due before the High Court this month, the unmarried mother's only source of income is the One-Parent Family Payment plus child benefit for her two children, aged 13 and 9, totalling €273 per week.

She accrued a debt of more than €18,000 to Monaghan Credit Union but in January 2004, after she failed to make the required repayments, the credit union secured an instalment order against her of €82 per week plus €282 legal costs.

Having failed to receive any of the instalments, the credit union went to Monaghan District Court in November 2005. The court ordered the arrest and imprisonment of the single mother for one month.

The woman's solicitor, Colin Daly of the Northside Community Law Centre, who took up the mother's case in January 2006, claims the imprisonment order is in conflict with the European Convention of Human Rights ? of which Ireland is a signatory ? which states: "No one person shall be deprived of their liberty merely on the grounds of inability to fulfil a contractual obligation."

The Irish Human Rights Commission (IHRC) has been joined as an interested party to the case and its president, Maurice Manning, said the case "raises an important issue around imprisonment for debt, which in these worsening economic times is likely to be an issue which will affect more and more people".