If you're one of the 4.5 million people who've yet to file an online self-assessment tax return and need to do so, act now to make sure you don't miss the deadline.

You have until 11.59pm on 31 January to send a complete online self-assessment tax return for the 2014/15 tax year, which ended on 5 April 2015, to HM Revenue & Customs.

If you miss the deadline you'll be fined £100. This applies even if you don't owe any tax or are due a refund.

It's even more important not to leave it until the last minute if you think you might have a query – in recent years call waiting times have doubled as the deadline approached.

The HMRC website has also crashed under the pressure of last minute returns. This year HMRC expects 980,000 returns to be filed on deadline day.

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 you haven't previously registered to complete returns online, be aware it can take HMRC up to seven working days to send you an activation code, so do this immediately. Not registering in time isn't a valid excuse for missing the deadline.

Do I need to file a return?

If you've been sent a self-assessment form, or received notification since April 2015 that you need to fill one in, then yes, you do. 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 2014/15 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 receive 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.

HMRC says that 11 million people are expected to file an online self-assessment tax return this year – and 4.5 million are still outstanding.

Filed your self-assessment tax return yet? Don't miss the 31 January deadline
Have you filed your self-assessment tax return? Don't miss the looming 31 January deadline

Can I file by paper instead?

The deadline for filing paper returns passed in October, so don't do it that way or you'll definitely be fined.

How do I file a return online?

To file online, you first need a login. If you've had one for previous years, it'll still be valid.

To register for a login for the first time, visit the HMRC website. Do this immediately – it can take up to seven working days for HMRC to post you the activation code you need, so if you do it later than Wednesday 20 January, it may not arrive in time.

What must I pay in my return?

As well as any 2014/15 tax owing, most self-assessment payers must also pay the first half of what's called a 'payment on account' for the 2015/16 tax year.

This is half the total expected tax due for 2015/16, which is estimated based on what you earned and paid in tax the previous year. So if you owe £2,000 for 2014/15, the first payment on account will be for £1,000, to be paid by 31 January 2016.

The next half (£1,000 in the example above) must be paid by 31 July, and come 31 January 2017, you'll settle the current year's tax bill, as you're doing now for 2014/15.

If you don't think you will earn as much money during 2015/16, you can ask for your payments on account to be reduced, but you will 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 now.

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?

If you want to pay your tax via bank transfer, you can do so right up until the evening of 31 January 2016. HMRC now 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.

There are other ways to pay, including debit or credit card. If you opt to pay this way HMRC will accept your payment on the date you make it, not the date it reaches HMRC's account – including on weekends.

What expenses can I claim?

For the self-employed, you only pay tax on profits after legitimate expenses, so claim back all you're entitled to (see HMRC's allowable expenses).

The deadline to register for self-assessment was 5 October. What if I missed it?

If you didn't register for self-assessment by 5 October you should contact HMRC immediately and explain your reasons for not registering in time. It will then assess whether or not you need to file a tax return. It is unlikely it will charge a penalty if you contact it and your excuse is reasonable.

However, if you fail to notify HMRC then you could face a penalty of up to 100% of any tax you're due to pay.

Can I still get an activation code if I ask for one within seven working days of 31 January?

If it gets within seven working days of 31 January and you've yet to apply for your activation code to file online, you should do it urgently.

You should also inform HMRC either online or by phone if you haven't yet got it, as if you have a genuine reason for not having been able to access your online account, HMRC may give you a few days extra (past the deadline of 31 January) to file your tax return. This is on a case-by-case basis and there is no guarantee HMRC will extend your deadline.

What if I'm late paying?

If you're late making your payment on account or tax bill you'll be charged 3% interest. This is on top of the minimum £100 fine for missing the deadline for filing your tax return.

I missed the 31 January deadline because my pet dog ate my tax return – will I still have to pay the £100 penalty?

Unsurprisingly, the answer is yes. The above is a real excuse one taxpayer used to appeal against a late filing penalty, according to HMRC. Other bizarre excuses include "my husband ran over my laptop", "my tax papers were left in the shed and the rat ate them" and "I had an argument with my wife and went to Italy for five years".

Ruth Owen, HMRC director general of personal tax, said: "We understand that life can be unpredictable and for those customers who have a genuine excuse for missing the 31 January deadline, such as the flooding, help is at hand. My advice would be to contact us through our helplines or online, as soon as possible.

"But for those who are trying to play the system, while the rest of us do the right thing, the message is clear: submit your tax return online by 31 January or face a fine."

However, if you have what HMRC considers to be a "reasonable excuse" for filing late, you can appeal against the penalty fine or the late payment interest.

Examples of a "reasonable excuse" include:

  • Your partner died shortly before the tax return or payment deadline.
  • You had an unexpected stay in hospital.
  • Your computer or software failed while you were preparing your online return.
  • Service issues with HMRC online services.
  • A fire prevented you from completing your tax return.
  • A disability that may have prevented you from filing on time.
  • Postal delays that you couldn't have predicted (although this only applies to applications made by post and the deadline for these closed on 31 October 2015).

If you wish to appeal, you should do so as soon as possible. See the HMRC website for how to do so. Bear in mind that appeals are considered on a case-by-case basis and there is no guarantee HMRC will cancel your penalty or late payment interest.

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).

You will need to discuss your financial position openly with HMRC and bring any outstanding tax affairs up to date.

What if I'm owed tax back?

After filing, you may be due cash back. This could happen if you've only worked part of the year, or you owe less than the total payments on account you've made for the previous tax year. See the refund forms for help claiming back.

How can I contact HMRC?

If you want to talk about your own tax affairs then you must contact HMRC by phone on 0300 200 3300. Expect long wait times, although HMRC says phone lines are less busy between 8.30am and 10.30am and 2pm and 4pm, Monday to Friday.

Call lines are not open weekends so don't leave queries to the last minute. If you have a general query you can contact HMRC by email or post.