Trade union the TUC has attacked the "complicated and confusing" letters sent to households which it said could lead to some people still entitled to child benefits opting out with "disastrous" consequences.
General secretary Frances O'Grady called on the Government to restore universal child benefit, as the deadline to register with HM Revenue and Customs ends tomorrow.
The chief executive of HM Revenue and Customs has warned parents to not leave registering for child benefits "until the last minute", on the eve of the stated deadline.
Lin Homer told BBC Radio 5 Live:
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.