An estimated 165,000 people missed Saturday's deadline, meaning they face penalties as well as losing the handout.
HM Revenue and Customs has urged those who have not registered to do so now to avoid further costs.
A spokesman for HMRC said : "More than 29,000 people registered for self-assessment over the weekend, taking the total registrations to 160,000.
"This means that 165,000 people still need to take action and on past experience we expect more people to register in the coming days.
"Although we are past the deadline, people should still register for self-assessment to minimise any penalties they may face."
Fines for failing to register will be decided on a case-by-case basis, HMRC said.
Under Government reforms, h ouseholds where one parent earns more than £60,000 a year have to return the entire amount through the self-assessment system unless they have opted out of receiving it in the first place.
It will be taken away on a sliding scale where mothers or fathers earn between £ 50,000 and £60,000.
HMRC chief executive Lin Homer said on Friday that twice as many families had opted out of receiving the payment than had been expected, and insisted the changes were going ''better than expected''.
But the Centre for Social Justice (CSJ) warned the cuts risked ''pouring further fuel on the fire'' of family breakdown.
Ms Homer told Radio 4's Today programme : ''I think we have been very proactive. We have written to 800,000 people. 1.2 million is the estimate and of course things like people's income changes during the year so we have written to everyone we know is affected by the change.''