Around 1.6 million people still need to submit their self-assessment tax returns by the end of Friday or face an automatic £100 penalty.
Meanwhile, a further 100,000 higher-earning parents who are liable for a tax charge because they receive child benefit have failed so far to register at all for self-assessment, HM Revenue and Customs (HMRC) said.
This group of parents do not have enough time left to meet Friday's deadline, but a spokesman for HMRC said they still have time to mitigate against any penalties if they "get their skates on" and register now.
A total of 10.9 million self-assessment returns are expected to be submitted for the 2012-13 tax year and last year a flurry of around 600,000 forms were sent in on deadline day.
The deadline for submitting an online self-assessment tax return and paying any tax due is midnight on Friday. Those in self-assessment who fail to meet this face an initial fixed penalty of £100, even if there is no tax to pay, followed by possible further charges.
Paper returns for the 2012-13 tax year should have been submitted by October 31 last year.
Households where one parent earns more than £50,000 a year who continued to receive child benefit after January 7, 2013 have to pay a tax charge. If one parent has a taxable income of more than £60,000, then they must pay all the money back.
Those parents affected by the changes were given the choice of either opting out of receiving child benefit altogether or carrying on receiving it and paying any charge at the end of each tax year under self-assessment.
But around 100,000 parents have neither registered for self-assessment nor opted out. Once they register, they will be sent a taxpayer reference which they can then use to get set up for self-assessment and file their return online. But this process will take longer than the Friday deadline.
As these people are not currently in the self-assessment system at all, they could face a "failure to notify" penalty, which would be based on the tax that should have been paid.
But the spokesman said that when looking at whether this penalty should be applied, the taxman will consider someone's circumstances on a "case-by-case basis", including what efforts they made to pay the tax.
He said: "Our staff will look at the facts of each individual case so someone who get their skates on now and registers for self-assessment will have a good case for not having to pay the failure to notify penalty."
Chas Roy-Chowdhury, head of taxation at the Association of Chartered Certified Accountants (Acca), suggested HMRC should give families who are struggling to get to grips with the self-assessment process "a lifeline" by extending Friday's deadline.
He said: "The circumstances around this year's deadline are different in that there will be a high number of people who will never have done self-assessment in their lives."
For those people who are registered for self-assessment, if a tax return is one day late, the penalty is £100. Extra daily penalties of £10 a day can then apply on top of this, up to a maximum of £900.
At six months if the return is still delayed, there is another penalty of 5% of the tax due or £300, whichever is more. After 12 months, there is a further penalty charge of 5% of the tax owing or £300, whichever is higher.
Previous research by HMRC found that Londoners are more likely to miss their tax return deadline than people living in any other part of the UK.
Around one in nine (11%) people living in inner London who had to send in a tax return last year failed to do so by the relevant deadlines.
The most punctual taxpayers live in the South West, with only 6% of their tax returns arriving late last year.