Better-off parents affected by child benefit cuts need to "get off their backsides" and register with HM Revenue and Customs, its head warned on the the eve of the stated deadline.
Families were given until the end of tomorrow to opt out of receiving the state help or register for income tax self-assessment so some or all of it can be clawed back - or face possible fines.
Officials say that in reality punishments are unlikely to be imposed on anyone who complies by the end of January, but HMRC chief executive Lin Homer said many thousands had still not done so.
More top news
A&E services at County Hospital in Stafford have been suspended for under 18s because the service is 'not clinically safe'.
GCSE grades have fallen across the board but girls continue to outperform boys, according to this year's results.
The serial killer Peter Sutcliffe has been moved from Broadmoor psychiatric hospital to prison.