There were a range of disclosure facilities, most of which have now closed. The Contractual Disclosure Facility is your remaining opportunity to disclose any tax irregularities to HMRC.
HMRC use the Contractual Disclosure Facility (CDF) in circumstances where they suspect serious tax fraud, but where they don't think that a criminal investigation is necessary or appropriate. Under the CDF, HMRC offers not to pursue a criminal investigation into any tax fraud which the taxpayer may disclose, provided that the taxpayer admits fraudulent conduct and makes a full disclosure of all irregularities in their affairs and the affairs of any entities for which they are responsible.
HMRC conduct an investigation under Code of Practice 9 with a view to the imposition of a civil penalty for fraudulent conduct if their suspicions are confirmed. The purpose of the enquiry is to determine the outstanding tax liabilities and interest and HMRC will impose civil penalties for fraudulent conduct if appropriate. HMRC will undertake the investigation with or without the taxpayer's voluntary co-operation. If the taxpayer decides to cooperate with HMRC, the investigation will proceed more quickly, efficiently and advantageously for both parties.