The Government has come under heavy criticism after two million families failed to register to collect child tax credits worth £45 a week.
The new benefit is mainly paid to mothers
Just three million of the five million plus families eligible for child credit and the new working tax credit had sent in claim forms by the end of February, according to Paymaster General Dawn Primarolo.
The Liberal Democrats have blamed the poor take-up on too many changes and too much form-filling.
"Any claim form that is 12 pages long with 47 pages of notes cannot be described as simple and straightforward," said Lib Dem spokesperson Steve Webb.
"The Government must ensure that families receive this money. Otherwise, it will not go from wallet to purse as intended, but from fathers' wallets straight into Gordon Brown's piggy bank."
The new child tax credit - announced during last year's budget - comes into effect on April 6.
The Government has spent £9m advertising the campaign that is part of its target to reduce the number of children in poverty by a quarter by 2005.
A Treasury spokesman said backdated payments would be available for several weeks for those who missed the April 6 launch of the new credit.
"We are extremely happy with what we have got so far and we would urge those people who have not claimed so far to do so," the spokesman said.
Under the new children's tax credit, the main carer - usually the mother - of the family can receive up to £545 in the 2003-04 tax year.
But higher national insurance payments and higher council taxes - together with unclaimed credits - could leave families worse off.