This year, 11.7 million taxpayers will need to file their self-assessment tax returns by 31 January - but with just one week until the deadline, around a quarter haven't got around to it, according to the latest data from HMRC.
Anyone who's self-employed, or who earns money from sources other than their main employment - including rent, pension or savings interest - could be required to file a tax return.
Here, we explain how to make sure your tax return is filed on time and what fines you could face if your tax return or tax payment is late.
You may need to submit a return if you earned untaxed income during the 2018-19 tax year, or in the following circumstances:
Despite the deadline being just a week away, only around 75% of eligible people - around 8.5 million taxpayers - have already completed and submitted their tax returns.
HMRC is urging the remaining three million people to complete their return as soon as possible in order to avoid any last-minute problems that could prove costly.
The amount of time it will take to complete your tax return will depend on how complicated your financial circumstances are, and whether you've organised your records and documents so that they're easy to find when you need them.
On average, people spent 3.4 hours completing their tax returns, according to Which? research. But, for a small number of people, the process took more than five hours.
The most time-consuming aspects tend to be finding receipts and records, and trying to understand the HMRC forms, according to our survey respondents.
However, using an online tool could cut down this time. Recent research from GoSimpleTax, our partners on the , found that its users took an average of 63 minutes and 21 seconds to complete their tax return from start to finish.
There are just a few days left to get your tax return to HMRC, but it's still doable - especially if you get yourself organised before you start. These simple steps could help:
Before you do anything, it's a good idea to get all of the documents you'll need together in the same place. That way, you won't be interrupted by having to leave your computer and hunt for them.
Your Unique Taxpayer Reference number, or UTR number, is necessary to file your self-assessment return, as it's personal to you and is used by HMRC to track your tax payments throughout your working life.
It stays the same every year, but if you've never filed before, you'll need to register for one. It can take up to 10 working days to arrive in the post, so if you haven't registered yet, the chances of getting this through in time to file on 31 January are slim - get in contact with HMRC as soon as possible if this is the case.
If you've forgotten your UTR, check last year's paperwork - or any other letters you've received from HMRC - as it can usually be found on there. It's 10 digits long and sometimes there's a letter 'K' at the end.
A lot of time can be wasted waiting for missing figures, but this is not an acceptable reason for submitting your return after the deadline.
Instead, you're better off estimating the figure and submitting your return on time - you can provide HMRC with the revised figures as soon as possible afterwards.
After you've submitted your return, you must keep your paperwork for a set period of time in case HMRC asks for proof of the numbers you've provided.
If your tax return doesn't reach HMRC by midnight on 31 January, you could face an instant fine of £100.
For each day late after that you're charged an additional £10, up to a maximum of £1,000.
If it's six months late, you're charged either £300 or 5% of the tax due (whichever is higher), on top of the penalties above.
If it's a year late, there's another £300 or 5% fine - but in some serious cases, you may be fined 100% of the tax due.
Your tax payment is also due on 31 January, so if your tax return is late, chances are that your tax bill will be late, too. This opens up a different set of charges - in addition to the charges for a late return.
Not only will you be charged interest from the date the payment was due (this is currently 3.25%), but there will be additional charges the later you are:
Some online tools and calculators can help shave some time off the submission process when time is tight.
The helps you pull together all of your income and outgoings, work out your tax liability and make suggestions for reliefs and allowances you might have missed, and you can use it to submit directly to HMRC.