Tax returns take a combined 19 million hours to complete - have you started yours yet?

Millions of people are yet to file their 2019-20 self-assessment tax return, with many expected to leave the task to the final day. But it could take much longer than you're anticipating.

A Which? survey found that self-assessors are collectively spending 19m hours working on their tax returns every year, based on the average amount of time survey respondents told us they spent on the task and the estimated number of people in the UK who file their own tax return.

Our survey also exposed some gaps in people's tax knowledge, as only three in 4,000 people correctly answered a series of true or false questions about self-assessment.

Here, Which? reveals the time it takes to complete the average tax return and how gaps in taxpayers' knowledge could mean people are paying more tax than they need to.

How long does it take to do a tax return?

Even if your tax knowledge is top-notch, doing your tax return can often take longer than you think.

According to our survey, the average time it takes to complete a tax return is two and a half hours.

While one in five speedy people said the process takes them less than an hour, for others it takes significantly longer - in fact, one in 10 respondents said that they spent more than five hours on the job.

To help speed up the process, it can be helpful to gather all of the documents you need before you start.

Depending on your circumstances, this will include things such as your National Insurance number, your UTR number, invoices, receipts, tenancy agreements, bank statements, P60 form, P45 form and student loan statements.

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

What happens if your tax return is late?

Normally the final deadline to file your tax return would be 31 January 2021and if you miss this deadline, you stand to be charged an immediate £100 penalty on 1 February.

However, on 25 January HMRC said it would not issue fines for tax returns submitted by 28 February. So if you fail to submit your return in February you will be hit with the £100 penalty on 1 March.

After the initial £100 fine, the fines will increase sharply:

• Up to three months late£10 fine for each day, capped at 90 days, plus the initial £100 fine
• Six months late An additional £300 or 5% of the tax due, whichever is larger
• 12 months late An additional £300 or 5% of the tax due, whichever is larger, although in serious cases you may be fined an additional 100% of the tax due.

While HMRC has announced it is waiving fines for late filers in February, the official deadline to pay your 2019-20 bill is still 31 January.

Your 2019-20 tax payment is due by midnight on 31 January, unless you have an alternative payment arrangement in place with HMRC.

Late tax payments incur separate charges. First, you'll be charged interest on the tax you owe from the date it was due; the current interest rate is 2.6%.

After 30 days, this will soon mount up:

• After 30 days (early March) A charge equal to 5% of outstanding tax
• After six months (31 July) Another 5% charge
• After 12 months (31 January) Another 5% charge.

Find out more:late tax returns and penalties for mistakes

How does your tax knowledge stack up?

When Which? posed seven true or false questions about self-assessment tax in its survey, just three of the 4,000 respondents got all of the questions right. Can you?

The question most of our survey respondents tripped up on was around who paid tax for interest earned in children's savings accounts - only one in six people got it right.

In a bid to stop parents using their child's savings account as a way to pay less tax themselves, there's the '£100 rule', which is applied to money paid into a child's account by parents, step-parents or guardians only - not grandparents, friends or other relatives.

Having an extra £100 added to your savings income may exceed your personal savings allowanceand could even tip you into a higher income tax band, so make sure you're careful about the money you've deposited.

Capital gains tax (CGT) was another area that caused some confusion. While a third of people correctly identified that you don't have to pay CGT when selling your home, around a third got the answer wrong and the remaining third said they didn't know.

The UK tax system is notoriously complicated, but it's important to know what your responsibilities are - failing to do so, even accidentally, could leave you with a hefty bill from HMRC.

Charges for mistakes on your tax return

In the worst cases, a lack of tax knowledge could lead to you making a mistake on your tax return that incurs a fine from HMRC - depending on whether it thinks you've been careless or purposely misleading.

You could face the following fines:

• No penalty You've taken 'reasonable care' to fill out your tax return
• 0 to 30% of tax owing You've been careless
• 20 to 70% of tax owing You have deliberately underestimated the tax you owe
• 30 to 100% of tax owing You've deliberately underestimated the tax you owe and have attempted to conceal the fact.

Submit your tax return with the Which? tax calculator

If you haven't submitted your tax return yet, consider trying the Which? tax calculator. This online tool is easy to use and totally jargon-free.

You can tot up your tax bill and it will suggest expenses you might have forgotten.

Then, when you're ready, it can even submit your return directly to HMRC.

This story was updated on 25 January 2021 with information about HMRC waiving fines for late filers in February.