Around two million people face a critical deadline this Saturday to submit their self-assessment tax returns for the 2024-25 tax year.
Missing the January 31 cut-off will trigger an automatic £100 penalty, with HM Revenue and Customs (HMRC) warning of further fines for continued delays.
HMRC confirmed over 10 million individuals had already completed their returns by Thursday. Over 12 million are expected to file a self-assessment for the current tax year.
An HMRC spokesperson said: "More customers have filed their tax return compared to this time last year, but for anyone yet to file and pay, don't leave it until January 31 - take action now.
"We encourage customers needing support to use our fast and easy digital and online services, and the majority already do. Should they still require help on deadline day, our telephone and webchat services will be available to provide additional support."
HMRC's phone lines are open Monday to Friday from 8am to 6pm.
The revenue body said that on Saturday its webchat service will have 10 times the usual Saturday capacity, with its 24/7 digital assistant and gov.uk guidance available.
It has enhanced this support with a phone service from 9am to 4pm, for people who need to speak to someone about self-assessment.
People who are unable to pay in full by January 31 may be able to set up a "time to pay" arrangement online if they owe less than £30,000 and meet criteria.
HMRC will consider customers' reasons for missing the deadline. Those with a reasonable excuse may avoid a penalty.
The initial £100 fixed penalty applies even if there is no tax to pay, or if the tax due is paid on time.
After three months, there could also be additional daily penalties of £10 per day, up to a maximum of £900.
And after six months, there could be a further penalty of 5% of the tax due or £300, whichever is greater.
After 12 months, there could be another 5% or £300 charge, whichever is greater.
There are also additional penalties for late payments of 5% of the tax unpaid at 30 days, six months and 12 months. If tax remains unpaid after the deadline, interest may also be charged on the amount owed.
Earlier in January, HMRC apologised after its helplines were down for part of a day due to a short-lived technical issue.
People should also watch out for scams, with criminals purporting to be from HMRC. Fraudsters may threaten people claiming they have an unpaid tax bill or they may make offers of fake rebates.
More information on how people can get in touch with HMRC is at http://www.gov.uk/find-hmrc-contacts/self-assessment-general-enquiries.
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