Nearly one in ten self-assessment taxpayers look set to miss today's deadline for paying HM Revenue & Customs, triggering fines of around £418 million.
An estimated 890,000 people who have to fill out a self-assessment tax return are expected either to get the form in late or make a mistake in calculating how much tax they owe, according to IFA Promotion.
The group, which supports the work of independent financial advisers, expects the figure to be slightly higher than the estimated 879,000 people who failed to make the deadline last year.
It claims people will be fined around £89 million for failing to return their form and the extra tax before the January 31 deadline, as well as paying £308 million in fines for miscalculations and £21 million in surcharges for unpaid tax from previous years.
The Revenue said around one million of the 9.5 million people who complete self-assessment tax returns failed to get their forms and the tax they owed in on time each year.
People who fail to meet the deadline face an automatic £100 fine, as well as having to pay interest on the tax they owe at 6.5 per cent.
If they have still not submitted their forms and tax six months later they will receive another £100 fine, while persistent offenders can face a fine of up to £60 a day.
Last year the Revenue started sending out shortened tax return forms of just four pages to people with simple affairs. These forms are designed to be completed by September 30 so that the Revenue calculates how much tax people owe.
A Revenue spokeswoman said: "This is the tenth year of self-assessment and the date shouldn't be a surprise.
"If you do miss it you do get a penalty and have to pay interest on any tax not paid."
David Elms, chief executive at IFA promotion, said: "To see that more money is set to be gifted to the taxman this year through fines for late returns and miscalculations is disappointing as it is entirely unnecessary."