Where credit card sales have been omitted from business takings, HMRC are encouraging taxpayers to come forward and make a disclosure of the income that has been omitted to avoid incurring interest and penalties on top of the unpaid tax.
As you may be aware HMRC now receive information from third parties such as banks and credit card companies and will then match that data with business accounts, and will then open detailed enquiries if the figures appear to be inconsistent. They can go back up to 20 years and the more serious cases can lead to criminal prosecution.
If you have other undeclared income or gains that don’t relate to credit card sales, there are other HMRC disclosure facilities to enable you to bring your tax affairs up to date.