Families receiving Child Benefit payments could be fined £100 at the end of the month if they forget to fill out a form.
New rules introduced last year mean families containing one adult earning more than £50,000 during the 2012/2013 tax year must repay part of their child benefit, while those earning more than £60,000 need to repay it completely.
Bruce Burrowes, who runs TaxAssistant Accountants, Merton Road, Wimbledon, has warned families they have until January 31 to file their tax return or they will be hit with an automatic £100 fine, with further penalties reaching £1,600 for those who file more than a year late.
Alternatively, families can opt out of Child Benefit, so they won't be liable for the new tax charge and won't need to file a tax return.
For those families choosing to opt out, Mr Burrowes advises they have their affairs in order, or risk losing their claim to state benefits later in life.
TaxAssistant Accounts, Wimbledon, estimate that 461,000 families have already opted out of receiving Child Benefit but about 120,000 have yet to act and are at risk of being fined.