Taxpayers who missed the January 2019 self-assessment deadline could see increased fines because of a delay to penalty notices.
Individuals who did not file their 2017/18 self-assessment tax return by the deadline at midnight on 31 January 2019 will receive an initial £100 penalty.
If they fail to submit their return after three months, additional penalties of £10 a day are incurred.
HMRC usually sends notices to inform late-filing taxpayers of these penalties in February, but this year it has said the letters will be delayed, with some arriving as late as the end of April.
The tax authority said the decision was made in order to deal with "an increased demand in our call centres as the UK leaves the EU".
The Association of Taxation Technicians (ATT) has raised concerns that because of the delay, some taxpayers may be left unaware that they are liable for additional fines.
Jon Stride, co-chair of the ATT's technical steering group, said:
"If the £100 penalty notice is issued by HMRC at the end of April 2019, a taxpayer may - by the time the notice hits their doormat - already be incurring additional penalties.
"Anyone who has yet to file their 2017/18 tax return should do so as a matter of urgency."
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