The tax credit system has been dogged with problems. It failed to deliver cash to pay packets and the helpline failed repeatedly.
Shadow cabinet minister David Lidington asked chancellor Gordon Brown to "list the time and duration of each occasion when staff have been unable to access the computer system".
Answering for the chancellor, Treasury minister Dawn Primarolo said, "The IT system has not been working as well as expected and there has been unscheduled downtime as a result. The Inland Revenue are working urgently to improve the system.''