It could take as long as six months for families to be paid child benefit.
Lib Dem activists earlier urged the government to keep child benefit payments for all mothers.
Child benefit, housing benefit and universal credit will be capped for two years from 2014-15.
The deadline has now expired for higher earners to opt out of receiving child benefit.
Significantly more than the recent withdrawal of child benefit from higher earners, for example.
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Changes to child benefit are "hugely popular" with most voters, a Treasury source has told the BBC.
This included child benefit, working families tax credit and income support, depending on their circumstances, it said.
In contrast working families have potentially lost tax credit, child benefit, pay rises and much more besides.
It has proposed the bill should be amended to remove child benefit from the calculations for household income.
It came as he defended his plans to cut spending and end child benefit for higher rate taxpayers.
But it also includes Child Benefit - currently paid to all parents who want it, regardless of income.
It meant 750, 000 more households would not now lose all their child benefit.
There will be a 10 euro per month reduction in child benefit rates.
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Prime Minister Tony Blair caused controversy when he suggested that parents of regular truants could lose child benefit payments.
As with the 10% tax rate, Mr Brown may announce next week how he intends to tax child benefit.
That includes three million who lose out only from the child benefit cap.
BBC: Welfare plans: Your benefits may be lower than expected
However, some families say "opting out" of child benefit is not an option.
BBC: Child benefit cuts lead thousands of parents to 'opt out'
One area facing possible cuts in the short-term are universal benefits such as child benefit and the winter fuel payment.
But the single biggest cut will come from the controversial removal of child benefit from families with a higher-rate taxpayer from 2013.
Some parents in Guernsey could lose their child benefit payments as the island's Social Security Department looks to reduce the benefit burden.
Her Majesty's Revenue and Customs has set up a Child Benefit Helpline on 0845 302 1444 for customers who want more details.
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Treasury minister David Gauke replied that the child benefit cut for higher earners would redirect money to those "who need it the most".
The changes to housing benefit and the benefit cap are the most obvious examples, or the removal of child benefit from higher income families.
Most working age benefits, such as Jobseekers Allowance and Child Benefit, will go up by 1%, less than the rate of inflation, for the next three years.
CTC, unlike the universal child benefit, requires means-testing and that a million families who have failed to apply for the credit may therefore miss out.
She told the government it was "not too late to change course", before urging ministers to commit to an "urgent" review of child benefit changes.
Two examples are child benefit, which all parents of dependent children receive, and winter-fuel payments, to which all households with someone over 60 are entitled.
You can have those with very high incomes, living abroad still able to claim child benefit, which highlights the cross-border difficulties in enforcing the policy.
The public are aware of imminent changes which could see more than a million people lose all or some of their child benefit, officials insist.
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