Better-off parents affected by child benefit cuts need to "get off their backsides" and register with HM Revenue and Customs, its head warned on the eve of the stated deadline to do so.
Families were given until the end of tomorrow to opt out of receiving the state help or register for income tax self-assessment so some or all of it can be clawed back - or face possible fines.
Officials say that in reality punishments are unlikely to be imposed on anyone who complies by the end of January, but HMRC chief executive Lin Homer said many thousands had still not done so.
"We think there are about 200,000 people who need to get off their backsides and do something," she told BBC Radio 5 Live.
"HMRC is committed to helping people pay the right amount of tax and urges parents who have been affected by the changes to child benefit to register for self-assessment," said Ms Homer.
"We know that lots of people leave it until the last minute."
Her comments came as the TUC attacked the "complicated and confusing" letters sent to households which it said could lead to some people still entitled to the benefit opting out with "disastrous" consequences.
Child benefit was withdrawn entirely from all families with at least one parent earning £60,000 a year or more from January, under plans set out by Chancellor George Osborne in last year's Budget.
Those on £50,000-plus have been stripped of 1% of the help for every £100 earned over that level to prevent a dramatic "cliff edge" loss.
The TUC said letters were being sent " arbitrarily to people unaffected by the change" and failed to make clear to parents what they needed to do.
It raised particular concerns over instructions related to ensuring stay-at-home parents did not lose out on credits required for them to qualify for a state pension.
General secretary Frances O'Grady called on the Government to restore universal child benefit and condemned the implementation of the policy as "a mess".
"Quite apart from the fact that the decision to withdraw the benefit is unfairly hitting single parents, many families where one parent earns more than £50,000 are - even at this late stage - still unaware that unless they've visited the HMRC website by the end of tomorrow they may be fined,"she said.
"To make matters worse, HMRC isn't only sending letters to those parents affected by the change.
"We've been hearing about parents earning below the £50,000 threshold who have been written to, as have mums and dads whose children are too old to qualify. Even childless couples have been receiving letters.
"There's also a risk that some vulnerable parents earning way below £50,000, and who get the HMRC letter by mistake, could think they are about to lose their child benefit, and rather than pay a fine, opt out of child benefit - with potentially disastrous consequences.
"Although registering for self-assessment only takes a matter of minutes, trying to work out whether to keep on receiving child benefit or stop the payments altogether is not easy. The complex changes and the confusing way in which affected parents have been informed makes it likely that many families where the mum or dad earns more than £50,000 may miss tomorrow's registration deadline."
An HMRC spokesman said that of the 1.1 million affected by the cut, 350,000 had opted out altogether, 425,000 people were already in self assessment and 140,000 had registered for it.
That left around 185,000 still to register.
"Failure to register for self assessment on time can result in a penalty depending on the circumstances," he said.
"The exact amount of any penalty would be on a case-by-case basis."
In response to the TUC concerns, HMRC said: "We have sent out two million letters and over the summer ran a comprehensive advertising campaign emphasising you had to have an income in excess of £50,000 per annum to be affected and letting people know exactly what they needed to do.
"People still have until midnight tomorrow to register for self assessment and we expect more to do so during the day."