Millions of Brits have just one day left to avoid getting £100 fine through the post

The clock is ticking for millions of Brits who want to avoid being slapped with a £100 fine from His Majesty's Revenue and Customs, or HMRC for short.

You see, today (31 January) is the deadline for anyone in the UK who needs to file their self-assessment tax return, which is the case for anybody who is self-employed or has a side hustle they've made money through.

That means even if you're employed by someone else and, don't need to file your own tax returns, but you've made over the tax-free annual allowance of £1,000 on the side, you'll still need to file a tax return.

Yes, that means if you've been selling off all of your old clothes on Vinted or whatnot and made more than that amount of money, you'll have to tell the taxman.

Nobody really loves paying taxes, so if you want to pay only what you owe and not a penny more, then you'd better get it done before the deadline.

Today's the last day to file your return with HMRC before you'll get a £100 fine. (Andrew Aitchison / In pictures via Getty Images)

That means you've only got today to sort things out before you risk the penalty fine of £100 for filing your return late.

If you're planning on leaving it even later than that, you'll rack up even more fines should you leave it more than three months after the deadline.

Alastair Douglas, CEO of TotallyMoney, said: "It's important to get your tax return filed as soon as possible - and even if there's nothing to pay. That's because the taxman will be waiting to dish out £100 fines as soon as the clock strikes midnight.

"After that, you'll have three months of breathing space before HMRC issues daily penalties of £10, up to a maximum of £900, with fines getting more severe after that."

After six months, you'll be fined five percent of the tax you owe, or £300 depending on which is higher, and after 12 months, you'll be hit with the same again.

If you keep being late the fines will go up, and millions of Brits left it until the last minute. (Peter Dazeley/Getty Images)

Failing to pay your taxes on time is a costly endeavour, as HMRC will also slap your tax bill with a 7.25 percent interest if you're late on your payments.

We can't really make it much clearer how bad an idea not paying your taxes on time is.

Just last week (23 January) the government said that two-thirds (8.6 million) of Brits who needed to file a self-assessment tax return had got their paperwork in.

However, that still left 3.4 million people still to get the details sorted out with just a few days left to go.

So, get on it, and keep that £100 for something nice.