Self-assessment taxpayers now have two weeks to complete and submit their returns for the 2017/18 tax year, before the deadline at midnight on 31 January 2019.
People who are self-employed or have more than one source of income are usually required to complete a self-assessment tax return.
31 January is also the deadline for taxpayers to make their first payment on account, and a balancing payment for any remaining tax owed.
Those who miss the deadline could face penalties starting at £100, even if they do not owe any tax.
With only 14 days to go, Financial Secretary to the Treasury Mel Stride warned "there are still many people that need to act now".
Angela MacDonald, director-general for customer services at HMRC, added:
"The self-assessment deadline on 31 January is fast approaching but there is still time for customers to file their tax returns online and on time to avoid any unnecessary penalties."
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