Child benefit would see cuts for the first two years of a Labour government, Ed Balls is expected to announce.
The shadow chancellor will present a 1% cap on rises in the help for parents as one of the "tough decisions" necessary to deal with the deficit if the party takes power next year - claiming it will save the taxpayer £400 million over five years.
He will seek to soften the blow by cutting ministers' pay by 5% and then freezing it until the party is able to "balance the books".
Addressing activists at the Opposition's final annual conference before the general election in May, he will vow not to "flinch from the tough decisions" needed to deal with the economy.
He will tell the Manchester gathering:
I want to see child benefit rising again in line with inflation in the next parliament.
But we will not spend money we cannot afford. So for the first two years of the next parliament we will cap the rise in child benefit at 1%.
It will save £400 million in the next parliament. And all the savings will go towards reducing the deficit.
Families would only be able to claim benefits for two children under plans put forward by a member of David Cameron's policy board aimed at cutting billions of pounds off the welfare bill, according to the Mail on Sunday.
The radical proposals reported to have been set out by Tory MP Nadhim Zahawi would limit child benefit and child tax credits to families' first two children.
According to the newspaper, he said capping benefits by family size would "save billions and help the next generation think more carefully about their relationship with the welfare state".
HM Revenue and Customs (HMRC) has urged parents who have not registered child benefits to do so now to avoid further fines.
This only applies to families where one parent earns £50,000 or more and who have not already opted out of the payments.
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.
Tens of thousands of parents could be fined for failing to register the child benefit they received this year with the taxman.
An estimated 165,000 people missed Saturday's deadline, meaning they face penalties as well as losing the handout.
Under Government reforms, households where one parent earns more than £50,000 a year have to return the benefits through the self-assessment system unless they have opted out of receiving it in the first place.
HM Revenue and Customs said the number of parents who had opted out of child benefits altogether had far exceeded expectation.
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.
Quite apart from the fact that the decision to withdraw the benefit is unfairly hitting single parents, many families where one parent earns more than £50,000 are - even at this late stage - still unaware that unless they've visited the HMRC website by the end of tomorrow they may be fined.
To make matters worse, HMRC isn't only sending letters to those parents affected by the change.
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:
We think there are about 200,000 people who need to get off their backsides and do something.
HMRC is committed to helping people pay the right amount of tax and urges parents who have been affected by the changes to child benefit to register for self-assessment.
We know that lots of people leave it until the last minute.
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.
Labour leader Ed Miliband has pledged to cap the overall welfare bill in a bold bid to regain the upper hand in the benefits debate.Read the full story ›
Labour MPs and Len McCluskey, the Unite General Secretary have put their support behind Ed Miliband on Twitter:
95p in every £ of public investment in Housing goes on Housing Benefit, only 5p on building homes. Labour will move from benefits to bricks!
Really pleased with significant week for Labour. Still lots to do, but important messages on fiscal probity and mutually fair welfare.
Liberal Democrat and Conservative MPs have taken to Twitter to blast Ed Miliband after his bid to regain the upper hand in the benefits debate by pledging to cap the overall welfare bill:
Could @ed_miliband just tell us ONCE why Labour did none of the things he is proposing in his/their 13 years in office?
When Ed Miliband says he wants to cap the overall welfare bill does he mean people in need get nothing when the money runs out?
Tax avoidance today, child benefit U-turn yesterday. Labour only stay the same when it comes to more borrowing and more debt.