Over 1.6m people - nearly one in five taxpayers (17.5%) - that submit paper returns are being urged by HMRC to send in their 2012/13 paper tax returns by 31 October to avoid a penalty.
Those unable to submit a paper return by the end of the month, can avoid a late-filing penalty by sending their return online by 31 January.
An HMRC spokesman said: 'As well as having an extra three months to file, if you file online your tax is calculated automatically and your tax return is processed faster, which means any tax refund you're due is repaid more quickly. Registering for HMRC online services is simple - just go to www.hmrc.gov.uk/online.'
Those sending a paper return close to the deadline, are urged to obtain proof of posting, as this provides useful evidence in the event of any appeal against a late-filing penalty.
The latest HMRC figures from last year, reveal that 7.93m (82.5%) on-time tax returns were sent online.