Britons who have received untaxed income or gains in the year ended 5 April 2009 need to complete a tax return before October 31 or risk a £100 penalty.

Accountants Grant Thornton predict that nine million Britons will have to fill in an HM Revenue & Customs tax return this year. This includes the self-employed and anyone who receives income from more than one source or who is a higher-rate taxpayer.

Francesca Lagerberg, head of tax at Grant Thornton, said: “The current economic climate has caused a growing number of people to seek additional income. Individuals may have rented out a property which has previously been used as a holiday or second home.

“This additional income would not be taxed at source and will require disclosure on an individual’s tax return, together with the obligation to notify HMRC by 5 October 2009.”

Any individuals who have not completed a tax return in the past and have therefore not been issued with a notice to file a return are also required to notify HMRC by 5 October 2009 if they have a new taxable income source where tax has not been accounted for at source.

“Just because you have not been sent a notification to complete a tax return does not automatically mean that you are not liable to complete one,” added Lagerberg.

“With the deadline of 5 October 2009 for notifying HMRC of the requirement to declare any untaxed income and gains received during the year ended 5 April 2009, now is the time to consider your affairs and take action where necessary”.