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Self-assessment: why join the 900,000 who file late and get a fine?

You've got the January blues so it must be time to fill in your self-assessment tax return. But that's something a lot of people can't face, with 10 per cent of Britain's nine million self-assessors expected to miss the 31 January deadline despite the prospect of a £100 fine.

But late entries are not the only headache for HM Revenue & Customs (HMRC) this year. Threatened strike action by staff in the run-up to the deadline could result in delays to the processing of both paper and online tax returns.

Chas Roy-Chowdhury, head of taxation for the Association of Chartered Certified Accountants, says: "The concern with the strike action is that your return or payment may make it to the HMRC office on time, but there may not be the appropriate people to process it. If you know you got everything in on time, but still receive a fine, appeal."

Some 150,000 self-assessment returns were received over the internet in the 24 hours running up to last year's deadline. During the filing peak, the HMRC website was processing more than 100 every minute.

Almost 2.9 million people filed online last year – a 40 per cent increase on the previous year. But be warned: if you plan to return your self-assessment form over the internet this year, you must register before 22 January in order to receive your registration and password details before 31 January.

If, despite a year's notice, you are one of the 900,000 expected to miss the deadline, it may not be a total disaster. Not all of those who file late will be subject to the £100 penalty charge as it is capped according to how much tax is actually due on that date. If, for example, you are late but owe no tax, typically because it is collected via your PAYE employment, then there would be no penalty for the delay. And if your reason for filing late is a good one, HMRC may waive the fine altogether.

"But at this stage, get your return in on time and get it right," warns Mr Roy-Chowdhury. "It will cost you more if there is a problem, so make sure it is as complete as possible and, if you are still waiting for a few documents, flag that up on your return form."