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
British Athletes are unlikely to achieve the same Olympic medal tally at Rio as they did at London in 2012, the head of UK Sport has said.
The strikebreakers would provide a "pool of relief staff" so the city could still operate during the Tube strikes.
A police officer faces a lawsuit after video emerged showing him handcuffing a young child with learning difficulties for 'misbehaving'.