Self-assessment: more than three million yet to file their tax return, with one week to go

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.

More than three million people yet to file tax returns

You may need to submit a return if you earned untaxed income during the 2018-19 tax year, or in the following circumstances:

• you're self-employed, a business partner, or a director of a limited company
• you're an employee or pensioner with an annual income of £100,000 or more
• you have a pre-tax investment income of £10,000 or more
• your income (or your partner's) was more than £50,000 and one of you claimed child benefit
• you received a P800 for from HMRC saying you didn't pay enough tax last year - and you didn't pay what you owe through your tax code or with a voluntary payment
• you have income from abroad that you need to pay tax on
• you were a trustee of a trust or a registered pension scheme
• you're a 'name' at Lloyd's of London
• you're a minister of religion
• you're a trustee or representative of someone who has died.

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.

• Find out more:how to fill in a self-assessment tax return

How long will my tax return take?

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 Which? tax calculator, found that its users took an average of 63 minutes and 21 seconds to complete their tax return from start to finish.

Tips for completing your tax return on time

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:

1. Get your documents together

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.

You'll need things such as your P60, P11D, PAYE notice of coding, P45, student loans statement, as well as receipts if you're claiming expenses, and a copy of your accounts if you're self-employed.

There's fairly extensive HMRC guidance that can let you know what you'll need.

Find your UTR number

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.

• Find out more:what is a UTR number?

Estimate income rather than file late

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.

Keep your paperwork

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 you're employed or a pensioner, you must keep your paperwork for at least 22 months from the end of the tax year; and for five years and 10 months if you're self-employed or letting property.

You might also need the paperwork if you're applying for a mortgage or loan, so it's worth keeping everything in a safe place - plus, it will make your life much easier when it comes to submitting your return next year.

What happens if you submit late?

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:

• After 30 days: a charge equal to 5% of the tax that's outstanding
• After six months: a further 5% charge
• After 12 months: a further 5% charge.

You can calculate your potential penalties using the gov.uk calculator.

• Find out more:late tax returns and penalties for mistakes

Get help filing your tax return with Which?

Some online tools and calculators can help shave some time off the submission process when time is tight.

The Which? tax calculator 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.