You have just days to submit your 2008/09 self assessment tax return on paper or risk a fine. And there's also a national postal strike to contend with.

The deadline to send paper returns to HM Revenue & Customs (HMRC) is Saturday 31 October (see the Latest Tax Rates guide).

Yet with a Royal Mail strike likely to take place on Thursday and Friday this week, you risk a late payment penalty of up to £100.

Royal Mail insists items sent via its special delivery service, which costs from £4.95 depending on weight, will make it on time.

However, if you don't want to take the chance, you can register to file online, meaning you have until 31 January 2010 to submit it.

No extension

There will be no extension to the paper deadline even if the strike takes place. HMRC only expects one million people to return their self assessment this way and says the majority have already done so.

An HMRC spokesman says if you can prove the strike caused the delay you may be able to appeal the fine, though your filing history will be considered to see if you have been late before.

Royal Mail is trying to limit disruption by recruiting 30,000 seasonal staff early to clear the anticipated backlog of mail, though it cannot guarantee a return will get to its destination in time, unless sent by special delivery.

Yet the special delivery next day guarantee will be withdrawn for items posted tomorrow, Thursday and Friday, but reinstated from Saturday.

More time to submit online

You have up to 31 January to file online. You must first register via the HMRC website, but will have to wait seven to 10 days to receive your full log-in details by post.

If you have completed your return online previously, you do not need to re-register.

If your paper return is late, you will be fined £100 and won't have the option to complete it online.

The 31 January cut-off is also the deadline to pay any tax due for 2008/09.

Who must send a self assessment return?

You must file one if you're self-employed, a company director or trustee, or if you have foreign income.

In addition, if you have complicated tax affairs, such as a second job, you may need to complete a tax return. If unsure, visit the HMRC website or call the self assessment helpline on 0845 900 0444.

If you've not filled in a tax return before but think you need to, you must let HMRC know.

If you pay tax via your employer's payroll, are not a director/trustee and don't earn additional income, you don't need to complete a return.

Get free tax help

If you're on a low income and can't afford professional help, the charity gives free advice on your rights and responsibilities.

Further reading/Key Links

How much to pay? Latest Tax Rates
Frequently asked tax questions: Taxing Times 6