More than 11.5 million taxpayers will need to file their self-assessment tax returns by 31 January - but with three weeks until the deadline, just under half haven't got around to it, according to HM Revenue & Customs (HMRC).
If you're self-employed, earned money from sources other than your salary this year, or are collecting a pension, you might be required to file a tax return.
Which? explains how to make sure your tax return is filed on time and avoid a fine.
HMRC estimates that 11.5m tax returns are likely to be due by the end of this month.
You may need to submit a return if you have untaxed income, or in the following circumstances:
At this stage, however, just 52% of eligible people - approximately 6.02m - have completed their returns and submitted them.
Of these, the vast majority of people filed online, through HMRC received 703,943 tax returns on paper (12% of the total).
This means that 5.54m are still to file their returns, with just three weeks until time runs out.
The amount of time it'll take you to file your return will depend on how complicated your circumstances are, and how well you've kept records.
On average, people spent 3.4 hours completing their tax returns, recent Which? research found. But a small portion (9%) spend more than five hours on their paperwork.
The most time-consuming aspects were finding receipts and records, and trying to understand the HMRC forms, according to our survey respondents.
Up to three months late, you'll pay £10 for each additional day, up to a maximum of £1,000. If you're six months late, you could pay either £300 or a penalty of 5% of the tax due.
And after 12 months, you could be fined up to 100% of the tax due, effectively doubling your bill.
You still have a few weeks before your tax return is due, but time is running out. Getting yourself organised in advance can save you time, and prevent last-minute stress.
As a first step, make sure you have all your important documents in one place, which might include your P60, P11D, your PAYE notice of coding and P45.
In order to submit yourtaxreturnonline, you'll need a . This stays the same every year, but if you've never filed before, you'll need to register for one - and it can take up to 10 working days to arrive in the post.
You'll then need to apply for an activation Pin, which could take another 10 days.So if this is your first time, make sure you register as soon as possible.
If you have a UTR, but have forgotten it, check last year's paperwork or get in contact with HMRC.
You might be chasing up missing figures, but don't risk missing the deadline.
Rather than submitting late, you're better off estimating the figure and submitting your return, then providing HMRC with the revised figures as soon as possible afterwards.
You can pull together all of your income and outgoings, work out your tax liability and submit your return directly to HMRC with a click of the button.