If a taxpayer has undeclared income or gains and is out of time to file a tax return and, there a no current disclosure amnesty/opportunity on-going they may volunteer the information to HMRC.

The reason one might volunteer information is to reduce or avoid tax penalties.

Phone HMRC's Voluntary Disclosure helpline on +44(0)300 1231078.

Post write to the Voluntary Disclosure Team at the following address: HM Revenue & Customs Individual and Small Business Compliance Campaigns Voluntary Disclosure Team S0790, Newcastle upon Tyne, NE98 1ZZ.

Use the Digital Disclosure Service to make an online disclosure.

Links: HMRC Voluntary Disclosure