Parents earning more than £50,000 who are still receiving child benefit have just a week to go to register for self-assessment.
Failing to do so by 5 October could land you with a hefty fine, HM Revenue & Customs warned this week.
There are now around 225,000 people who need to register by next Saturday, the taxman said.
However, not all who miss the deadline will be penalised. HMRC said it will take individuals' intentions into account on a case-by-case basis when deciding penalties.
Act now, said Anita Monteith, a manager in the Tax Faculty at the Institute of Chartered Accountants in England and Wales: "The new High Income Child Benefit Charge adds considerable complication to the tax system for ordinary people, particularly for those brought into the self -assessment system for the first time.
"I would urge those affected to register for self-assessment as soon as they can. The more prepared you are for self-assessment, the easier it will be."
To check whether the tax charge applies and to register, go online at hmrc.gov.uk/childbenefitcharge.
If you have to complete a tax return, you will need to provide details of all your income as well as details of the child benefit to be clawed back. You will also need the latest P60 given to you by your employer, as well as details of any bank interest received.