Taxpayers who have failed to submit tax returns for past years have one week left to come forward and take advantage of an HM Revenue and Customs' (HMRC) campaign.
Launched on July 9, the campaign is aimed at people who received a Self Assessment tax return, or a notice to complete a tax return for any year up to and including 2011-12, but have failed to submit it.
To benefit from the campaign terms and to avoid a higher penalty, participants have until October 15 to complete and submit their outstanding tax returns, and pay the tax and National Insurance Contributions that they owe.
After October 15, penalties of up to 100 per cent of the tax, or even criminal investigation, could follow.
Marian Wilson, HMRC's head of campaigns, said: "People who take part in this campaign will receive the best terms available - but they need to send returns without delay to meet the October 15 closing date.
"I urge people to take advantage of the quickest and most straightforward way to bring their tax affairs up to date and benefit now from the best terms available rather than face a higher bill later."
HMRC campaigns launched so far have produced more than £553 million by people coming forward voluntarily to pay the tax they owe. HMRC analyses information using a state-of-the-art software system, Connect, to identify people who have gaps in their tax records.
This campaign follows last year's Tax Return Initiative, which covered higher-rate taxpayers who had failed to submit 2008-09 or 2009-10 returns.
As a result of the campaign, more than £30 million was paid to HMRC in outstanding tax, with over 3,000 people coming forward voluntarily, filing more than 5,500 tax returns. After an opportunity focusing on higher rate taxpayers closed last year, HMRC issued £58 million in tax demands to those who did not come forward.