Nearly two million people have still to file their self-assessment tax returns before tomorrow's deadline, Sky News has learned.
Although a total of 8.8 million returns have so far been filed, a further 1.8 million must get their online returns in before Thursday's midnight deadline to avoid being hit with an automatic penalty.
The £100 fine for filing of the self-assessment document on February 1 or later is imposed even if workers have already paid any tax due for the year or no tax is liable.
An HM Revenue and Customs (HMRC) spokesman told Sky News: "If you still haven't sent in your tax return, you've got until tomorrow to file to avoid a penalty – and please also make sure to pay what you owe."
The Tax Office said that on Tuesday a total of 321,000 people filed the returns - at a rate of 13,375 an hour.
In recent years HMRC has bolstered its online data facilities to try and keep pace with online filing.
There has been a near-doubling of people filing tax returns over the internet during the past five years.
In January 2008, 45.9% of the 9.3 million returns were filed online and the figure topped 80.9% of 10.5 million returns by last January.
HMRC recently revealed that on Christmas Day 1,548 people also filled in forms while another 4,685 people filed on Boxing Day.
On January 31, 2012 a technical fault left deadline day self-assessment taxpayers unable to check their payment status.
Sky News understands the fault was caused by outside payment service providers and not internal Tax Office systems.
The glitch occurred as HMRC was inundated with returns on deadline day.
Days later HMRC said that a total of 90.4% of taxpayers had met that self-assessment deadline, by filing online or on paper forms.