Fines for missing the 31st January deadline start at £100.

The ACCA said that changes to child benefits and a rise in the number of self-employed workers mean there will be more first-time self-assessors struggling to meet the deadline.

Chas Roy-Chowdhury, ACCA head of taxation, said: “HMRC has a common-sense decision to make. Either it can stick to the deadline and penalise all those families and self-employed people who are struggling to get to grips with the self-assessment process, or it can do the right thing and give them a lifeline by extending the deadline. Self-assessment is not easy and there are fines starting at £100 for missing the deadline even if you don’t owe any tax.

“The circumstances around this year’s deadline are different in that there will be a high number of people who will never have done self-assessment in their lives. They are going to miss the deadline not because they have been putting it off, but because they are newcomers. If reports are to be believed that 2 million have yet to meet the deadline just two days before it closes, then it is likely a lot of people will miss it.”

Many families will be completing the self-assessment returns for the first time because of changes to the child benefit system which means any parent who earns between £50,000 and £60,000 can elect to continue to receive child benefit but they must complete a self-assessment form and will have to repay a proportion of the benefit they receive, by way of a tax charge, at the end of the tax year.

Failure to complete a self-assessment could result in having to repay part of or all of the benefit claimed by way of a tax charge on the highest earner of the couple, but also interest and penalties on the tax unpaid, even if it is an innocent mistake.

Roy-Chowdhury added: “It’s not an easy form to complete and often mistakes will happen because people filling them in don’t know for sure what should be included. For example, interest gained on mis-sold payment protection insurance compensation needs to be declared, but it is not obvious. Get it wrong and you could be fined.

“We urge HMRC to do the right thing and postpone the deadline.”

If you miss the 31 January deadline you will be fined £100. From then on, the fine will be £10 per day for the next 90 days. Meaning if you are three months late in filing and even if you owe no tax you will have a fine of £1000.