Time is running out for millions of taxpayers to file their online self-assessment tax return by the end of the month. Act NOW to avoid a £100 fine for missing the deadline – especially if you're filing online for the first time, as it could take up to 10 days for first-time filers to receive an activation code.

If you're among the 3.5 million taxpayers who need to file a return and have yet to do so, you have until 11.59pm on Tuesday 31 January to send HMRC a complete online self-assessment tax return for the 2015/16 tax year, which ended on 5 April 2016.

Update Tuesday 31 January: Today’s the deadline for filing your self-assessment tax return. This story was first published earlier this month but the info in it still stands – though you may no longer have time to request a new password. If you're struggling to log-in and you're worried you may miss the deadline then make sure you contact HMRC ASAP.

You need to be registered to complete your return online. (This is separate to registering for self-assessment itself, which you should have done by 5 October.)

If this is your first time filing a return then you'll need to register for a login – visit the HMRC website to register. It's crucial you do this ASAP as it can take up to 10 days for you to receive the activation code you need – which means you need to request one by Saturday (21 January) at the latest to be sure of getting one in time. If you've filed a return previously, you should be able to use the same login again.

What if I've lost my login/password details?

With so many different logins to remember for various accounts, it's understandable that some people may have forgotten their HMRC password info.

If that's the case for you, there are two ways you can retrieve your password:

  • Online: By far the quicker option, but HMRC can only reissue your password online if you can provide the email address it holds for your 'online Government account'. If you've forgotten this – you set up a special email address for your tax that you no longer remember, say – you'll need to contact HMRC's call centre to explain what's happened. Act quickly as it can take days to get replacements sorted.
  • By post: If this is the option you plump for, you should make sure you leave yourself plenty of time, as it could take seven days for the password to arrive.

Can I file a paper return instead?

The deadline for filing paper returns was 31 October – so you must file your return online to avoid paying a penalty. If you were to file a paper return now you would be fined.

Have you filed your self-assessment tax return yet? You've got until 31 January
Miss the deadline and you'll be fined £100 – this applies even if you don't owe any tax or are due a refund

What must I pay in my return?

As well as any 2015/16 tax owing, most self-assessment payers must also pay the first half of what's called a 'payment on account' for the 2016/17 tax year.

This is half the total expected tax due for 2016/17, which is estimated based on what you earned and paid in tax the previous year. So if you owe £2,000 for 2015/16, the first payment on account will be for £1,000, to be paid by 31 January 2017.

The next half (£1,000 in the example above) must be paid by 31 July 2017, and come 31 January 2018, you'll settle 2017/18's tax bill, as you're doing now for 2016/17.

If you don't think you'll earn as much money during 2016/17, you can ask for your payment on account to be reduced, but you'll have to give a valid reason, such as an expected drop in profits or a change in circumstances.

You only have to make payments on account if your previous year's tax came to more than £1,000. But if an employer (if you have more than one job) has already deducted more than 80% of that figure, you won't owe a payment on account.

HMRC will usually send you a self-assessment statement that shows how much you owe, or you can check your tax bill online.

How can I pay?

You can pay by bank transfer, debit or credit card (although credit card payments will incur a fee). HMRC will accept your payment on the date you make it, not the date it reaches HMRC's account – including on weekends.

If you want to pay your tax via bank transfer, you can do so right up until the evening of 31 January (but you'll be cutting it fine if you wait until then).

HMRC accepts money under the Faster Payments system, which allows cash to go through in two hours. However, each bank has a limit on how much you can transfer under Faster Payments. The limits range from £5,000 to £100,000. See each provider's limit.

What if I miss the deadline?

You'll be charged a £100 penalty if you fail to submit your return by the deadline, with further penalties of £10 a day applied after three months. If you pay late, you'll be charged an extra 2.75% of whatever you owe.

The Government provides an online tool for calculating how much you'll need to pay in penalties and interest if you miss the deadlines.

What if I can't afford to pay the tax?

If your bill is correct but you can't afford to pay it, contact HMRC as soon as possible as you may be able to avoid late payment penalties by coming to an arrangement to spread your payments over a period of time (see our Free Tax Code Calculator to ensure you're on the right tax code).

A reasonable excuse for not paying your tax on time is normally something unexpected or outside your control that stopped you meeting a tax obligation, for example:

  • Your partner or another close relative died shortly before the tax return or payment deadline.
  • You had an unexpected stay in hospital that prevented you from dealing with your tax affairs.
  • You had a serious or life-threatening illness.
  • Your computer or software failed just before or while you were preparing your online return.
  • Service issues with HMRC online services.
  • A fire, flood or theft prevented you from completing your tax return.

HMRC says whether or not it allows taxpayers to avoid late payment penalties "depends on individual circumstances". If you're struggling to pay before the deadline you should call 0300 200 3835.

Do I have to file a self-assessment tax return?

Don't be alarmed if this is the first you've heard of the self-assessment tax return deadline, as the chances are you don't need to file a return.

This form of self-assessment is typically for people who are self-employed, although there are other instances where you might need to complete an online return (these are listed below).

If you've been sent a self-assessment form, or received notification since April 2016 that you need to fill one in, you'll need to file an online return by Tuesday 31 January. But if you haven't received a form, there are still some special circumstances that mean you could need to file a return.

If your tax is deducted by your employer, you usually don't need to submit a form unless you get additional income from a second job or freelance work, or have been caught up in the changes to child benefit.

Since 7 January 2013, all parents with incomes above £50,000 who receive child benefit payments have to pay a tax charge based on their income, as well as how much of the benefit they received in the 2015/16 tax year.

If HMRC has asked you to complete a tax return but you don't think you need to, tell it as soon as possible. You'll have to pay a penalty if you simply don't send one. If you haven't received notification, you should get in touch with HMRC if you fall into one of the following categories, as it's likely you'll need to file a return:

  • You're self-employed.
  • You're a partner in a business partnership.
  • You're a company director.
  • Your annual income is £100,000 or more.
  • You have income from property.
  • You have income from savings or investments that has been taxed and was £10,000 or more before you paid tax on it.
  • You have income from savings or investments that hasn't been taxed and is £2,500 or more (bank account interest is usually taxed automatically).
  • You need to claim expenses or reliefs.
  • You or your partner receives child benefit and your income is more than £50,000.
  • You get income from overseas.
  • You have income from trusts, settlements or estates.
  • You have capital gains tax to pay.
  • You've lived or worked abroad or don't live in the UK permanently.
  • You're a trustee.

It's worth mentioning though that if you owe a one-off or a small amount you may be able to make an individual payment to HMRC, rather than filing an online return.

Who should I contact for further advice?

Official help and advice on completing a self-assessment form is available online and you can use Twitter to get general help from HMRC, if you start tweets with @HMRCcustomers.

You can also call the HMRC helpline on 0300 200 3310. This is open 8am to 8pm on weekdays, 8am to 4pm on Saturdays and 9am to 5pm on Sundays.