HM Revenue and Customs (HMRC) has issued a warning, urging people to make a “quick check” to avoid missing out on £100 of their hard-earned cash. With millions set to file a self-assessment tax return in the new year, some people may not actually realise they need to do so.

And anyone who is required to and fails to meet the January 31 deadline may be hit with a £100 penalty – even if no tax is owed. The deadline to register for self-assessment is this Saturday, October 5, and HMRC has sent out an alert to those who may be unaware they need to file a return.

In an update on X, formerly known as Twitter, HMRC said: “Do you need to do a Self Assessment tax return? You can quickly check using our free online tool.”

According to Gov.uk guidance, you must send a tax return if in the last year, April 6 2023 to April 5, 2024, you were self-employed and earned more than £1,000, were a partner in a business, had a total taxable income of more than £150,000, or had to pay Capital Gains Tax when you sold or “disposed of” something that increased in value. You’ll also need to file a return if you had to pay the High Income Child Benefit Charge, and you may need to send one if you have any untaxed income such as money from renting out a property, tips and/or commission, income from savings, investments, and dividends, or foreign income.

HMRC said it “urges customers to file their return early to provide peace of mind and to also allow time to consider opportunities to spread the cost of their tax bill, claim refunds earlier and avoid costly errors caused by rushing.”

If you miss the October 5 deadline to register, there is a possibility you may eventually be charged a penalty for failing to notify HMRC that you owe tax, according to the Low Incomes Tax Reform Group. This is called a “failure to notify” penalty and is calculated as a percentage of the tax that is owed.

If you notify HMRC late, you can usually avoid a failure to notify penalty if you can both file your 2023/24 tax return online and pay any income tax owed for the 2023/24 tax year by January 31. However, if you do not register for self-assessment, you’re more likely to forget to file the return altogether – leading to a £100 fine, and potentially other penalties on top.