235,000 Child Benefit claimants on higher incomes need to register with HMRC by midnight on 5th October or face fines.
After that they will need to fill in self-assessment tax returns, to be returned on paper by the end of October or by 31st January 2014 if submitted online.
HMRC says that 1.1m were caught by the new policy of removing Child Benefit progressively, through a tax charge, from those on between £50,000 and £60,000 a year and completely from those on over £60,000.
405,000 have simply opted out of the benefit. Others are already caught up in the self-assessment process or have recently registered.
But 235,000 have only 11 days to register or risk fines ranging up to 100% of the extra tax they owe. As Radio 4's Moneybox highlighted over the weekend, the fines could be many hundreds of pounds, in theory.
Child Benefit is paid at the rate of £20.30 a week for the first child and £13.40 for each subsequent one, so a family with two children receives £1,752 a year.
In reality, HMRC is only likely to impose 100% fines on deliberate tax-dodgers.
And there is a way to redeem yourself, even if you fail to register by the 5th October deadline.
If you still pay the extra tax due, through the self-assessment system, the penalty is likely to be offset against your tax - in other words, it reduces to zero.
Here's the government website which tells you how to register. It's a simple process - the tax forms come later.