Millions of taxpayers have until 31 January to file their online self-assessment tax return. Act NOW to avoid a £100 fine for missing the deadline – especially if you're filing online for the first time or don't know your login details.

If you need to file a self-assessment tax return there are two things you need to do by 11.59pm on Wednesday 31 January:

  • File a complete online self-assessment tax return. This is for the 2016/17 tax year.
  • Pay your tax bill. You'll need to pay any remaining tax you owe for 2016/17, plus in many cases a payment towards what you owe for 2017/18.

Remember, as of last Saturday you CAN'T pay your tax bill with a personal credit card – HM Revenue & Customs has stopped accepting them as a result of card fees being banned.

What login details do I need to complete my return online?

To complete your return online, you need to be registered with Government Gateway or Verify (you should be if you've filed a return previously). This is separate to registering for self-assessment itself, which you should have done by 5 October. You can sign in on the website to file your return.

If it's your first time filing a return, you'll need to register for a login – again, via Do this ASAP as it can take up to 10 days to receive the activation code you need.

What if I've lost my user ID/password details?

With multiple logins to remember for different accounts, it's understandable that some people may forget their HMRC user ID or password info.

If you've forgotten your user ID OR your password, here's how to retrieve it:

  • Online: By far the quicker option, but HMRC can only reissue your user ID or password online if you can access the email address it holds for you.
  • By post: In this case, your user ID or password will be sent to you – leave plenty of time, as it could take seven days to arrive.

If you've forgotten your user ID AND password (it happens...) you'll need to go through a longer reset process with HMRC which will require extra info such as your national insurance number and takes about 10 minutes.

You can reset your user ID and password and read more info on the sign in or register page.

Can't I just file a paper return instead?

Not at this stage. The deadline for filing paper returns was 31 October, so you must file your return online – if you were to file a paper return now you would be fined.

What do I need to pay with my tax return?

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

As well as any 2016/17 tax owing, most self-assessment payers must also pay the first half of what's called a 'payment on account' for 2017/18.

This is half the total expected tax due for 2017/18, estimated based on what you earned and paid in tax the previous year. So if you owe £2,000 for 2016/17, the first payment on account will be for £1,000, to be paid by 31 January 2018.

The next half (£1,000 in the example above) must be paid by 31 July 2018, and if you've anything left to pay for 2017/18, you'll have to do so by 31 January 2019.

If you don't think you'll earn as much money during 2017/18, 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 won't have to make payments on account if your previous year's self-assessment tax bill came to less than £1,000, or if you've already paid more than 80% of the total tax you owe – eg, you have more than one job and your employer has deducted tax as normal from your earnings.

How can I pay?

You can pay by bank transfer or debit card – as of 13 January 2018 you can no longer pay HMRC by personal credit card. 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 by 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 using Faster Payments, ranging from £5,000 to £250,000. See each provider's limit.

What if I miss the deadline?

You'll be fined – but exactly how much depends on what you fail to do by the deadline:

  • If you fail to file your return on time... you'll be charged a £100 penalty. If you're more than three months late, you'll be charged a further penalty of £10 a day for a maximum of 90 days.
  • If you fail to pay your tax bill on time... you'll be charged a £50 penalty plus 3% interest on what you owe. If you're more than six months late, you'll be charged a further £50 penalty.

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 find you can't afford 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).

You'll need a reasonable excuse for not paying your tax on time. This is usually 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.
  • 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.

If you've been sent a self-assessment form or received notice since April 2017 that you need to fill one in, you'll need to do so – it's typically required of self-employed people, but there are a few other instances when you might need to do it too.

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 2013, all parents with incomes above £50,000 who receive child benefit payments have to pay a tax charge based on their income.

If HMRC has asked you to complete a tax return but you don't think you need to, don't ignore the request, and 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.
  • You receive a state pension and it was more than your personal allowance and was your only source of income – unless you started getting your pension on or after 6 April 2016.

It's worth mentioning though that if you owe a one-off or 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 tweet @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.