Tens of thousands of parents have only days left to claim a tax allowance which could be worth more than £1,000.
The last chance of a slice of the little-known Children's Tax Credit Relief will disappear for good on January 31.
Parents could be forgiven for not knowing about the payment as it lasted for only two years before being replaced by the Child Tax Credit.
It is available to households which included children under 16 between April 6, 2001, and April 5, 2003.
For taxpayers entitled to the full relief, it is worth £529. If a child was born during that period - more than a million were - there is another allowance worth £520.
Unusually, the Children's Tax Credit Relief was not means-tested so every family could apply for it, regardless of income.
But it was limited to one payment per family per tax year, regardless of the number of children.
The January 31 deadline comes in because HM Revenue & Customs has strict rules on claiming allowances, which can be backdated for only five years.
That means the 2007/08 tax year is the last in which CTCR could figure, and January 31 is the deadline for filing returns.
The Revenue says the majority of people who were entitled to the relief have already claimed it, but claims-handling firms say they are receiving thousands of inquiries.
Adam Thomsett, director at claims-handler challengeyour.com, said: 'There are going to be an awful lot of people who aren't aware of this relief and you can't really blame them.
'It was buried in the mire of all the countless reliefs, credits and allowances the Government has introduced and then withdrawn over the years. But it's money that's rightfully theirs.'
Mark Wallace, campaign director of the Taxpayers' Alliance, said: 'People have been put off claiming these allowances because the Government gets it wrong and comes back to them years later saying they've underpaid their tax. So take-up is never great. But these can be valuable allowances that should be taken up by all families.'
A spokesman for the Revenue said: 'Although we haven't advertised it since it was replaced it was well publicised at the time.'
Few claimants will be given actual cash, however. Most will receive an extra allowance to reduce the income tax they have to pay.
• The Inland Revenue says the quickest way to claim is to download a copy of the claim form from www.hmrc.gov.uk/forms/11ctc.pdf
You can also visit your local tax office or inquiry centre where staff will download the form for you to fill in immediately.
You can find the nearest centre in the phonebook.
The self- employed can make a claim on their tax return. Those who have filed this already may be entitled to a rebate. Employees need to fill in form 11CTC. http://forums.moneysavingexpert.com/showthread.html?t=1191059