Last-minute extension for self-assessment deadline - for some

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Tax Time spelled out on a calculator.

It's the final day for filing your tax return. You have until midnight tonight to get it in, and still up to 1.5 million people have failed to submit their return yet. HMRC has said that it expects around 600,000 forms on the last day, which leaves just under a million who will be liable for a £100 fine for late filing.

By now you may be resigned to paying a £100 fine at the very least - but there could be a last-minute reprieve on the cards.

Tax experts at Tolley have found that for one category of people, there's another 15 days before the fine applies.

Do you qualify?

When you use the free HMRC software to file your return online, you might assume that once you have your documents prepared, you can go online, file your return, and pay up a few minutes before the deadline.

However, there's an extra step which can trip you up. When you first register with the software you cannot file your return until you have a user ID, password and activation code, which will be sent in two separate letters through the post.

The extension applies to those people who registered for the first time - and realised their mistake - sometime between midnight on 21 January and midnight tonight. It also applies to those who have forgotten their user ID and password and apply for a new one in this time frame. These groups will have until 15 February to file their return.

This is confirmed by the automatic email that is sent to you when you register or request a replacement, which says: "If you enroll for the Self Assessment online service or request a replacement User ID or password between midnight 21 and midnight 31 January GMT, you must file your tax return online before midnight 15 February to avoid receiving a penalty. You must still pay any tax due by midnight 31 January." The Guardian has received confirmation from HMRC that this is the case.

Unfortunately, if you haven't yet registered with HMRC as having to complete a tax-return - and don't have a UTR number yet - then you won't get any extension.

Paying

It's worth highlighting that this doesn't exclude you from paying anything you owe by the 31 January deadline. You'll need to pay as usual, it's just the accompanying paperwork that can wait another 15 days.

Tolley has warned that the automatically generated penalty letters are unlikely to be able to distinguish between those who are late and those who have had their deadline extended, so it says you should appeal the charge within 30 days. You should print off the automatic email you received and use it as part of your appeal.

If you qualify for the extension, this will come as a relief because the fines can be shocking. They start at £100 - due the second your return is late. However, if you let your return slip three months beyond the deadline, the fines start mounting alarmingly - at a rate of £10 a day until they hit £900.

If you let it drag past 6 months you'll get another fine, and 12 months after the deadline you'll be fined again. In total you can end up paying well in excess of £1,000 for missing the deadline - and HMRC has the power to fine you even more than this if they feel you are deliberately being difficult.

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