If you receive a letter referring to COP9, it means you’re being given the opportunity to make a complete and accurate disclosure of all deliberate and non-deliberate conduct that have led to suspected irregularities in your tax affairs.
If HMRC suspects that a full disclosure is not made, they reserve the right to trigger a full criminal investigation with a view to prosecute.
‘Deliberate conduct’ means the taxpayer knew that an entry or entries included in a tax return or accounts were wrondg, but the taxpayer submitted them anyway, or that the taxpayer knew a liability to tax existed but chose not to tell the HMRC at the right time.
During the course of an investigation, if the taxpayer makes materially false or misleading statements, or provides materially false documents, HMRC reserves the right to start a separate criminal investigation into that conduct. This is separate from the misleading tax investigation.
Note: By the time HMRC approaches you, they probably already have the bulk of the evidence. They may be looking to see if you will continue that hole or find a way to cover it.