HMRC has warned over a million Brits they face a £100 fine if they ignore a yearly letter.
Next week 1.5 million people in the UK will have to submit their tax returns - or risk a hefty penalty.
The deadline for submitting tax returns was January 31, but, this year HMRC gave customers extra time to complete their 2020/21 tax return.
They have until February 28 to file their return to avoid a late filing penalty.
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About 12.2million customers are expected to file a tax return for 2020/21 tax year and more than 10.2million were received by January 31.
But about 1.5 million customers have just one week left to complete their late tax return.
The government charges a £100 penalty if you miss the self-assessment filing deadline – even if there's no tax to pay.
Further penalties of £10 a day are then applied after three months, up to a maximum of £900.
If you miss the payment deadline, or don’t set up a payment plan, you’ll be charged five per cent of the unpaid tax after 30 days, six months and 12 months.
But interest of 2.75 per cent on outstanding payments will still accrue from February 1 - although they won't kick in until 30 days after the 31 January deadline.
HMRC has given customers until April 1 to pay their outstanding tax bill or set up a 'time to pay' arrangement to avoid receiving a late payment penalty.
The taxman has been charging interest on all outstanding balances since February 1, says the Mirror.
The time to pay service allows any individual or business who needs it the option to spread their tax payments out into manageable monthly instalments.
Self assessment taxpayers with up to £30,000 of tax debt can do this online once they have filed their return.
Almost 100,000 customers have used this service since April last year.
If customers owe more than £30,000, or need longer to pay, they should call the Self Assessment Payment Helpline on 0300 200 3822.
HMRC director general for customer services Myrtle Lloyd said: “There is one week left to complete your tax return if you haven’t done so already.
"And for anyone who is worried about paying their tax bill, there is support available – search ‘pay my Self Assessment’ on gov.uk.”
From today, February 22, customers will be able to make self assessment tax payments through the HMRC app.
You can either connect to their bank to make these payments or pay by direct debit, personal debit card or corporate and commercial credit or debit cards.
