The department says it is essential people plan well ahead because it can take up to seven days for the activation code - essential to using the online system - to be sent through the post.
Anyone who missed the paper deadline for the 2011-12 tax year - October 31st - must send their return to HMRC online by January 31st next year.
Additionally, any tax that is owed by the taxpayer must have reached the department by the same date, making it essential to plan well ahead.
For those failing to submit their return on time, an initial £100 fixed penalty will be issued, irrespective of whether there is any tax to pay.
This will steadily increase the longer it takes for the return to reach HMRC, meaning it is best to act now.
Posted by Thomas Fletcher